U.S. patent application number 11/771903 was filed with the patent office on 2008-08-07 for request detection error processing.
This patent application is currently assigned to ViaSat, Inc.. Invention is credited to Donald W. Becker.
Application Number | 20080186994 11/771903 |
Document ID | / |
Family ID | 39535615 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080186994 |
Kind Code |
A1 |
Becker; Donald W. |
August 7, 2008 |
REQUEST DETECTION ERROR PROCESSING
Abstract
Methods and apparatuses are presented for conducting
communications over a shared communication medium, involving (a)
sending a request from a first node, the shared communication
medium organized to include (i) a request signal space including a
plurality of request segments each having a different location
within the request signal space and (ii) a scheduled transmission
signal space including a plurality of scheduled transmission
segments each having a different location within the scheduled
transmission signal space, the request sent in a request segment,
(b) if no assignment is received from a second node in response to
the request, retransmitting the request from the first node, and
(c) if an assignment is received from the second node in response
to the request, associating the request with a scheduled
transmission segment selected from the plurality of scheduled
transmission segments, sending a data transmission from the first
node in the scheduled transmission segment associated with the
request.
Inventors: |
Becker; Donald W.; (Rancho
Santa Fe, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW LLP;VIASAT, INC (CLIENT #017018)
TWO EMBARCADERO CENTER
EIGHTH FLOOR
CA
94111
US
|
Assignee: |
ViaSat, Inc.
Carlsbad
CA
|
Family ID: |
39535615 |
Appl. No.: |
11/771903 |
Filed: |
June 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60895143 |
Mar 16, 2007 |
|
|
|
60888451 |
Feb 6, 2007 |
|
|
|
Current U.S.
Class: |
370/431 |
Current CPC
Class: |
H04B 7/18582 20130101;
H04W 72/1273 20130101; H04W 84/06 20130101; H04W 72/1284
20130101 |
Class at
Publication: |
370/431 |
International
Class: |
H04L 5/00 20060101
H04L005/00 |
Claims
1. A method for conducting communications over a shared
communication medium involving a plurality of nodes, the method
comprising: (a) sending a request from a first node in the
plurality of nodes, the shared communication medium organized to
include a request signal space and a scheduled transmission signal
space, the request signal space including a plurality of request
segments each having a different location within the request signal
space, the scheduled transmission signal space including a
plurality of scheduled transmission segments each having a
different location within the scheduled transmission signal space,
the request sent in a request segment that is one of the plurality
of request segments; (b) if no assignment is received from a second
node in response to the request, retransmitting the request from
the first node; and (c) if an assignment is received from the
second node in response to the request, wherein the assignment
associates the request with a scheduled transmission segment
selected from the plurality of scheduled transmission segments,
sending a data transmission from the first node in the scheduled
transmission segment associated with the request in accordance with
the assignment.
2. The method of claim 1 wherein no assignment is received from the
second node in response to the request, wherein the request is not
detected by the second node.
3. The method of claim 1 wherein no assignment is received from the
second node in response to the request, wherein the request is
detected by the second node as being involved in a collision.
4. The method of claim 1 wherein the second node fails to detect a
collision involving the request and sends the assignment to the
first node.
5. The method of claim 4 wherein the first node sends a data
transmission in response to the assignment from the second
node.
6. The method of claim 5 wherein the data transmission is involved
in a collision with another data transmission.
7. The method of claim 1 wherein receipt of the data transmission
is not to be acknowledged by the second node, and if the second
node does not properly receive the data transmission, the data
transmission is not re-transmitted.
8. The method of claim 1 wherein receipt of the data transmission
is to be acknowledged by the second node, and if the second node
fails to send an acknowledgement of receipt of the data
transmission, the first node sends a second request to request a
second scheduled transmission segment for re-sending the data
transmission.
9. A method for conducting communications over a shared
communication medium involving a plurality of nodes including a
first node and a second node, the method comprising: (a) at the
second node, receiving a re-transmitted request from the first
node, the shared communication medium organized to include a
request signal space and a scheduled transmission signal space, the
request signal space including a plurality of request segments each
having a different location within the request signal space, the
scheduled transmission signal space including a plurality of
scheduled transmission segments each having a different location
within the scheduled transmission signal space, the re-transmitted
request sent in a request segment that is one of the plurality of
request segments; (b) at the second node, making an assignment
associating the re-transmitted request with a scheduled
transmission segment selected from the plurality of scheduled
transmission segments, the assignment taking into account location
of the re-transmitted request within the request signal space, and
sending a corresponding assignment message; and (c) at the second
node, receiving a data transmission from the first node in the
scheduled transmission segment associated with the re-transmitted
request.
10. An apparatus for conducting communications over a shared
communication medium involving a plurality of nodes, comprising:
(a) a first node capable of sending a request over the shared
communication medium, the shared communication medium organized to
include a request signal space and a scheduled transmission signal
space, the request signal space including a plurality of request
segments each having a different location within the request signal
space, the scheduled transmission signal space including a
plurality of scheduled transmission segments each having a
different location within the scheduled transmission signal space,
the first node capable of sending the request in a request segment
that is one of the plurality of request segments; (b) wherein if no
assignment is received from a second node in response to the
request, the first node is capable of re-transmitting the request;
and (c) wherein if an assignment is received from the second node
in response to the request, wherein the assignment associates the
request with a scheduled transmission segment selected from the
plurality of scheduled transmission segments, the first node is
capable of sending a data transmission in the scheduled
transmission segment associated with the request in accordance with
the assignment.
11. The apparatus of claim 10 wherein if no assignment is received
from the second node in response to the request, because the
request is not detected by the second node, the first node is
capable of re-transmitting the request.
12. The apparatus of claim 10 wherein if no assignment is received
from the second node in response to the request, because the
request is detected by the second node as being involved in a
collision, the first node is capable of re-transmitting the
request.
13. The apparatus of claim 10 wherein the first node is capable of
receiving the assignment from the second node, wherein the second
node fails to detect a collision involving the request and sends
the assignment.
14. The apparatus of claim 13 wherein the first node is capable of
sending the data transmission in response to the assignment from
the second node.
15. The apparatus of claim 14 wherein the data transmission is
involved in a collision with another data transmission.
16. The apparatus of claim 10 wherein receipt of the data
transmission is not to be acknowledged by the second node, and if
the second node does not properly receive the data transmission,
the data transmission is not re-transmitted.
17. The apparatus of claim 10 wherein receipt of the data
transmission is to be acknowledged by the second node, and if the
second node fails to send an acknowledgement of receipt of the data
transmission, the first node is capable of sending a second request
to request a second scheduled transmission segment for re-sending
the data transmission.
18. An apparatus for conducting communications over a shared
communication medium involving a plurality of nodes, comprising:
(a) a second node capable of receiving a re-transmitted request
from a first node, the shared communication medium organized to
include a request signal space and a scheduled transmission signal
space, the request signal space including a plurality of request
segments each having a different location within the request signal
space, the scheduled transmission signal space including a
plurality of scheduled transmission segments each having a
different location within the scheduled transmission signal space,
the second node capable of receiving the re-transmitted request in
a request segment that is one of the plurality of request segments;
(b) wherein the second node is capable of making an assignment
associating the re-transmitted request with a scheduled
transmission segment selected from the plurality of scheduled
transmission segments, the assignment taking into account location
of the re-transmitted request within the request signal space, and
sending a corresponding assignment message; and (c) wherein the
second node is capable of receiving a data transmission from the
first node in the scheduled transmission segment associated with
the re-transmitted request.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application claims benefit of priority under 35
U.S.C. 119(e) of U.S. provisional Application No. 60/888,451, filed
on Feb. 6, 2007, entitled "Symbol Oriented Reservation Request
System For Controlling Access To A Shared Communications Link,"
Attorney Docket No. 017018-014000US, Client Reference No.
VS-0239-US, the content of which is incorporated herein by
reference in its entirety.
[0002] The present application also claims benefit of priority
under 35 U.S.C. 119(e) of U.S. provisional Application No.
60/895,143, filed on Mar. 16, 2007, entitled "Blip and Burst
Multiple Access," Attorney Docket No. 017018-014400US, Client
Reference No. VS-0242-US, the content of which is incorporated
herein by reference in its entirety.
[0003] The following U.S. nonprovisional patent applications,
including the present application, are being filed concurrently and
the disclosure of every other application is incorporated by
reference in the present application in its entirety for all
purposes: [0004] U.S. Nonprovisional patent application Ser. No.
______, filed ______ for "Reservation Request System for
Controlling Access to a Shared Communication Medium" (Attorney
Docket No. 017018-014410US) (Client Reference No. VS-0242-US-2).
[0005] U.S. Nonprovisional patent application Ser. No. ______,
filed ______ for "Time Multiplexed Requests for Controlling Access
to a Shared Communication Medium" (Attorney Docket No.
017018-015100US) (Client Reference No. VS-0253-US). [0006] U.S.
Nonprovisional patent application Ser. No. ______, filed ______ for
"Code Multiplexed Requests for Controlling Access to a Shared
Communication Medium" (Attorney Docket No. 017018-015200US) (Client
Reference No. VS-0254-US). [0007] U.S. Nonprovisional patent
application Ser. No. ______, filed ______ for "Contention and
Polled Requests for Scheduling Transmissions" (Attorney Docket No.
017018-015300US) (Client Reference No. VS-0255-US). [0008] U.S.
Nonprovisional patent application Ser. No. ______, filed ______ for
"Request Signal Designs for Multiple Service Types" (Attorney
Docket No. 017018-015400US) (Client Reference No. VS-0256-US).
[0009] U.S. Nonprovisional patent application Ser. No. ______,
filed ______ for "Default Assignment of Scheduled Transmissions"
(Attorney Docket No. 017018-015500US) (Client Reference No.
VS-0257-US). [0010] U.S. Nonprovisional patent application Ser. No.
______, filed ______ for "Successive Scheduled Requests for
Transmission" (Attorney Docket No. 017018-015600US) (Client
Reference No. VS-0258-US). [0011] U.S. Nonprovisional patent
application Ser. No. ______, filed ______ for "Piggyback Requests
in Scheduled Transmissions" (Attorney Docket No. 017018-015700US)
(Client Reference No. VS-0259-US). [0012] U.S. Nonprovisional
patent application Ser. No. ______, filed ______ for "Robust and
Efficient Assignment of Scheduled Transmissions" (Attorney Docket
No. 017018-015800US) (Client Reference No. VS-0260-US). [0013] U.S.
Nonprovisional patent application Ser. No. ______, filed ______ for
"Request Detection Error Processing" (Attorney Docket No.
017018-015900US) (Client Reference No. VS-0261-US). [0014] U.S.
Nonprovisional patent application Ser. No. ______, filed ______ for
"Assignment Modes for a Shared Communication Medium" (Attorney
Docket No. 017018-016000US) (Client Reference No. VS-0262-US).
[0015] U.S. Nonprovisional patent application Ser. No. ______,
filed ______ for "Reservation Request Improvements" (Attorney
Docket No. 017018-016100US) (Client Reference No. VS-0263-US).
BACKGROUND OF THE INVENTION
[0016] In many applications, a communication medium is shared among
a number of nodes. The nodes compete with one another for access to
the shared communication medium. At any given moment, there may be
more than one of the nodes that wish to transmit data over the
shared communication medium. A system is typically put in place to
facilitate access to the shared communication medium by the various
nodes. Various categories of such multiple access systems have been
developed.
[0017] One category of multiple access systems utilizes contention
protocols. Examples of these contention protocols include the ALOHA
protocol and the slotted ALOHA protocol, which are known in the
art. Here, each node is allowed to freely transmit its data over
the shared communication medium at any time or any slotted time. In
a system employing a hub, each node sends its transmission to the
hub, which then broadcasts the transmission to all nodes. In a
system without a hub, each node directly broadcasts its
transmission to all nodes. In either case, every node listens to
the channel for its own transmission and attempts to receive it. If
a node is unsuccessful in receiving its own transmission, the node
can assume that its transmission was involved in a collision with
another transmission, and the node simply re-transmits its data
after waiting a random amount of time. In this manner, collisions
are allowed to occur but are resolved by the nodes.
[0018] Another category of multiple access systems utilizes carrier
sense protocols. Examples include persistent carrier sense multiple
access (persistent CSMA) and non-persistent carrier sense multiple
access (non-persistent CSMA) protocols, which are known in the art.
Generally speaking, these protocols require each node to listen to
the shared communication medium before transmitting. Only if the
shared communication medium is available is the node allowed to
transmit its data. In persistent CSMA, when a node senses that the
shared communication medium is not available, the node continually
listens to the shared communication medium and attempts to transmit
as soon as the medium becomes available. In non-persistent CSMA,
when a node senses that the shared communication medium is not
available, the node waits an amount of time before attempting to
listen to the shared communication channel for an opportunity to
transmit. Even though a node listens first before transmitting,
there still exists a probability for collisions. This is because
when the medium is available, two or more nodes can detect the
availability and decide that they are going to transmit data.
Various techniques have been developed to handle such
collisions.
[0019] Yet another category of multiple access systems utilizes
contention free protocols. Here, each node can reserve the shared
communication medium in order to transmit data. The node can
transmit data without colliding with transmissions from other
nodes. This is because the shared communication medium is reserved,
for a particular time duration for example, for the node's
transmission and not for any other transmission. A significant
advantage of contention free protocols is that the communication
medium is not taken up by unsuccessful transmissions that collide
with one another and resulting re-transmission attempts. This can
lead to a more efficient use of the shared communication medium,
especially as the number of nodes and number of data transmissions
increase.
[0020] However, contention free protocols require a reservation
process that allows nodes to reserve use of the shared
communication medium. Making such reservations also requires
communications. If the reservation process itself occupies too much
of the shared communication medium, performance of the system can
be negatively impacted. Thus, to take full advantage of the
benefits of contention free multiple access, more efficient systems
for reservation of the shared communication medium are needed.
BRIEF SUMMARY OF THE INVENTION
[0021] The present invention relates to methods and apparatuses for
conducting communications over a shared communication medium
involving a plurality of nodes, involving (a) sending a request
from a first node in the plurality of nodes, the shared
communication medium organized to include a request signal space
and a scheduled transmission signal space, the request signal space
including a plurality of request segments each having a different
location within the request signal space, the scheduled
transmission signal space including a plurality of scheduled
transmission segments each having a different location within the
scheduled transmission signal space, the request sent in a request
segment that is one of the plurality of request segments, (b) if no
assignment is received from a second node in response to the
request, retransmitting the request from the first node, and (c) if
an assignment is received from the second node in response to the
request, wherein the assignment associates the request with a
scheduled transmission segment selected from the plurality of
scheduled transmission segments, sending a data transmission from
the first node in the scheduled transmission segment associated
with the request in accordance with the assignment.
[0022] In one embodiment, no assignment is received from the second
node in response to the request, and the request is not detected by
the second node. In another embodiment, no assignment is received
from the second node in response to the request, and the request is
detected by the second node as being involved in a collision.
[0023] The second node may fail to detect a collision involving the
request and send the assignment to the first node. The first node
may send a data transmission in response to the assignment from the
second node. Here, the data transmission may be involved in a
collision with another data transmission.
[0024] According to one embodiment of the invention, receipt of the
data transmission is not to be acknowledged by the second node, and
if the second node does not properly receive the data transmission,
the data transmission is not re-transmitted. According to another
embodiment of the invention, receipt of the data transmission is to
be acknowledged by the second node, and if the second node fails to
send an acknowledgement of receipt of the data transmission, the
first node sends a second request to request a second scheduled
transmission segment for re-sending the data transmission.
[0025] The present invention further relates to methods and
apparatuses for conducting communications over a shared
communication medium involving a plurality of nodes including a
first node and a second node, involving (a) at the second node,
receiving a re-transmitted request from the first node, the shared
communication medium organized to include a request signal space
and a scheduled transmission signal space, the request signal space
including a plurality of request segments each having a different
location within the request signal space, the scheduled
transmission signal space including a plurality of scheduled
transmission segments each having a different location within the
scheduled transmission signal space, the re-transmitted request
sent in a request segment that is one of the plurality of request
segments, (b) at the second node, making an assignment associating
the re-transmitted request with a scheduled transmission segment
selected from the plurality of scheduled transmission segments, the
assignment taking into account location of the re-transmitted
request within the request signal space, and sending a
corresponding assignment message, and (c) at the second node,
receiving a data transmission from the first node in the scheduled
transmission segment associated with the re-transmitted
request.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 presents a simplified network including a scheduler
node 102 and a plurality of access nodes 104, 106, 108, and 110
utilizing a shared communication medium, according to one
embodiment of the invention.
[0027] FIG. 2 depicts a time-division multiplexing scheme as
applied to a frequency channel having a bandwidth of 32 Hz over a
duration of 1 second.
[0028] FIG. 3 depicts a frequency division multiplexing scheme as
applied to a frequency channel having a bandwidth of 32 Hz over a
duration of 1 second.
[0029] FIG. 4 depicts a wavelet-division multiplexing scheme as
applied to a frequency channel having a bandwidth of 32 Hz over a
duration of 1 second.
[0030] FIG. 5 is an illustrative signal diagram showing time
division multiplexing (TDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with TDM
request segments and scheduled transmission segments, according to
an embodiment of the invention.
[0031] FIG. 6 is an illustrative signal diagram showing a
two-service TDM request and scheduled transmission scheme, in
accordance with an embodiment of the invention.
[0032] FIG. 7 is an illustrative signal diagram showing orthogonal
frequency division multiplexing (OFDM) as utilized to partition the
request signal space and the scheduled transmission signal space,
with OFDM/TDM request segments and scheduled transmission segments,
according to an embodiment of the invention.
[0033] FIG. 8 is an illustrative signal diagram showing another
example of orthogonal frequency division multiplexing (OFDM) as
utilized to partition the request signal space and the scheduled
transmission signal space, with OFDM/TDM request segments and
scheduled transmission segments, according to an embodiment of the
invention.
[0034] FIG. 9 is an illustrative signal diagram showing frequency
division multiplexing (FDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with
FDM/TDM request segments and scheduled transmission segments,
according to an embodiment of the invention.
[0035] FIG. 10 is an illustrative signal diagram showing time
division multiplexing (TDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with
synchronous CDM request segments, according to an embodiment of the
invention.
[0036] FIG. 11 is an illustrative signal diagram showing time
division multiplexing (TDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with
quasi-synchronous CDM request segments, according to an embodiment
of the invention.
[0037] FIG. 12 is an illustrative signal diagram showing code
division multiplexing (CDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with CDM
request segments and CDM scheduled transmission segments, according
to an embodiment of the invention.
[0038] FIG. 13 presents a more detailed example of a system with a
contention code request channel, according to an embodiment of the
invention.
[0039] FIG. 14 presents a more detailed example of a system with a
polled code request channel, according to an embodiment of the
invention.
[0040] FIG. 15 is a plot of expected envelop detector performance
under different noise levels, according to an embodiment of the
invention.
[0041] FIG. 16 depicts use of piggyback requests for request
collision detection and missed request processing, according to an
embodiment of the invention.
[0042] FIG. 17 presents a simplified network operating under a "no
scheduler mode," according to one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The present invention relates to communications conducted
over a shared communications medium involving a plurality of nodes.
The invention is specifically related to techniques employed for
assigning opportunities for transmission based on requests.
[0044] FIG. 1 presents a simplified network including a scheduler
node 102 and a plurality of access nodes 104, 106, 108, and 110
utilizing a shared communication medium, according to one
embodiment of the invention. This configuration corresponds to a
mode of operation referred to here as "scheduler mode," which is
described as an illustrative example.
[0045] Referring to FIG. 1, scheduler node 102 serves to control
usage of the shared communication medium by access nodes 104, 106,
108, and 110. The shared communication medium can represent any
communication medium that may be utilized by more than one node.
For example, the shared communication medium can represent signal
space in one or more satellite channels. Thus, the access nodes and
the scheduler node may be part of a satellite network. As another
example, the shared communication medium can represent signal space
in one or more wireless terrestrial channels. Thus, the access
nodes and the scheduler node may be part of a terrestrial wireless
network. As yet another example, the shared communication medium
can represent signal space in one or more wired channels. Thus, the
access nodes and scheduler node may be part of a wired network.
[0046] Furthermore, embodiments of the present invention may be
implemented in different network topologies that involve a shared
communication medium. These may include star topologies, mesh
topologies, bus topologies, and others.
[0047] In the present embodiment of the invention, scheduler node
102 provides control over access of the shared communication medium
by access nodes 104, 106, 108, and 110. In order to transmit data
over the shared communication medium, an access node such as access
nodes 104, 106, 108, and 110 first send a request to scheduler node
102. In response, scheduler node 102 assigns an opportunity for
data transmission to the access node. Scheduler node 102 sends an
assignment message associated with the assignment to the access
nodes. Upon receiving the assignment, the access node that made the
request can transmit data in the assigned transmission opportunity.
This general scheme of request, assignment, and transmission is
used in various embodiments of the invention. However, as noted
before, other embodiments of the invention may involve variations
and different operations.
Symbols
[0048] Generally speaking, a basic unit of data transmission is
referred to here as a "symbol." A symbol can be defined to have one
out of a number of possible values. For example, a binary symbol
may have one of two possible values, such as "0" and "1." Thus, a
sequence of N binary symbols may convey 2.sup.N possible messages.
More generally speaking, an M-ary symbol may have M possible
values. Thus, a sequence of N M-ary symbols may convey M.sup.N
possible messages.
[0049] The concept of symbol and the methods by which a symbol can
assume values is quite general. In many applications, a symbol is
associated with a defined baseband pulse shape which is
up-converted to a carrier frequency with a particular phase
relationship to the carrier and with a particular amplitude. The
amplitude and/or phase of the symbol is known as the modulation and
carries the information of a symbol. The set of permissible
modulation points defined in the amplitude and phase plane is known
as the modulation constellation. The amount of information that a
symbol may convey is related to the number of discrete points of
the constellation. 16-QAM is an example of amplitude-phase
constellation which allows transmission of up to 4 bits of
information per symbol. In some applications, only the phase is
used for modulation. Quadra-phase shift keying (QPSK) is an example
of pure phase modulation which allows transmission of up to 2 bits
of information per symbol. In other applications, the symbol
waveform may be defined such that symbol phase may either not exist
or be difficult to receive accurately, in which case pure amplitude
modulation can be used. One example of binary amplitude modulation
is on-off amplitude-shift keying modulation which allows
transmission of up to 1 bit of information per symbol.
[0050] Each symbol may occupy a particular portion of the relevant
signal space. Specifically, each symbol may be said to occupy a
certain amount "time-bandwidth product." Here, an amount of
time-bandwidth product is a scalar quantity that may be measured in
units of Hz-seconds and does not necessarily dictate how the signal
is distributed within the signal space. In theory, symbols cannot
be strictly limited in both time and frequency. It is customary,
however, to define the time-bandwidth product of a signal to be the
time-bandwidth product of the region in which the preponderance of
signal energy resides. Since precise definitions of time-bandwidth
product vary somewhat throughout the literature, figures showing
symbol boundaries in time-frequency space should only be considered
as approximate representations.
[0051] Just as a simple example, a signal spanning a bandwidth of 1
Hz and lasting a duration of 1 second may have a time-bandwidth
product of 1 Hz-second. A signal spanning a bandwidth of 0.5 Hz and
lasting a duration of 2 seconds may also have a time-bandwidth
product of 1 Hz-second. Similarly, a signal spanning a bandwidth of
0.1 Hz and lasting a duration of 10 seconds may also have a
time-bandwidth product of 1 Hz-second. These examples do not assume
any multiplexing of the signal space, which is discussed separately
below. Also, the particular values used in these example and other
examples described herein are for illustrative purpose only.
Different values may be used in actual systems.
[0052] The measurement of a symbol in terms of an amount of
time-bandwidth product is also applicable when different signal
space multiplexing techniques are employed. Such techniques may
include time-division multiplexing, frequency-division
multiplexing, wavelet-division multiplexing, code-division
multiplexing, and/or others. In each of the following four
examples, a symbol occupies a time-bandwidth product of 1
Hz-second, even though different signal space multiplexing
techniques are used.
[0053] In a first example, FIG. 2 depicts a time-division
multiplexing scheme as applied to a frequency channel having a
bandwidth of 32 Hz over a duration of 1 second. The channel is
divided into 32 time slots, each having a duration of 1/32 second.
A symbol may be transmitted in each 1/32-second time slot over the
bandwidth of 32 Hz. In this example, each symbol has a
time-bandwidth product of 1 Hz-second.
[0054] In a second example, FIG. 3 depicts a frequency division
multiplexing scheme as applied to a frequency channel having a
bandwidth of 32 Hz over a duration of 1 second. The channel is
divided into 32 different frequency sub-channels each having a
bandwidth of 1 Hz. A symbol may be transmitted in each 1 Hz
frequency sub-channel over the duration of 1 second. In this
example, each symbol also has a time-bandwidth product of 1
Hz-second.
[0055] In a third example, FIG. 4 depicts a wavelet-division
multiplexing scheme as applied to a frequency channel having a
bandwidth of 32 Hz over a duration of 1 second. The channel is
divided into 32 different time and frequency symbol segments. 2
symbol segments have a bandwidth of 1 Hz with a duration of 1
second, 2 other symbol segments have a bandwidth of 2 Hz with a
duration of 1/2 second, 4 other symbol segments have a bandwidth of
4 Hz with a duration of 1/4 second, 8 other symbol segments have a
bandwidth of 8 Hz with a duration of 1/8 second, and 16 additional
symbol segments have a bandwidth of 16 Hz with a duration of 1/16
second, In this example, each symbol has a time-frequency product
of 1 Hz-second, as well.
[0056] In a fourth example, a code-division multiplexing scheme is
applied to a frequency channel having a bandwidth of 32 Hz over a
duration of 1 second. For this example, it is assumed that there
are 32 different possible orthogonal code words, each comprising a
unique 32-chip binary pattern. Each code word represents a unique
"code channel." To send a symbol on a particular code channel, the
symbol value is used to modulate the code word associated with the
code channel, and the resulting signal is sent. In the case of
binary phase shift keying (BPSK) symbols, for instance, a symbol
having a value of "1" may be sent by simply sending the code word,
and a symbol having a value of "0" may be sent by sending the
inverted version (180-degree phase shift) of the code word. The 32
symbols sent using 32 different "code channels" are
non-interfering, and as a group, they occupy a common 32 Hz by 1
second portion of the time-frequency space. In this example, each
symbol has an effective time-frequency product of 1 Hz-second.
Symbol-Level Request
[0057] Referring back to FIG. 1, a symbol-level request may be sent
from an access node such as access nodes 104, 106, 108, and 110
according to an embodiment of the present invention. Here, a
symbol-level request refers to a request that can be sent in the
form of a transmission signal having a time-bandwidth product
comparable to that of a symbol. For example, a symbol-level request
may occupy exactly one symbol. Thus, a protocol message comprising
a large number of symbols, representing a header and a data payload
that must be processed and interpreted, would not be considered a
symbol-level request.
[0058] The novel use of a symbol-level request according to
embodiments of the present invention allows for highly efficient
utilization of the available signal space. Because of its compact
size, a symbol-level request may not have sufficient capacity to
carry a significant data payload. However, according to various
embodiments of the invention, information may be conveyed in the
choice of the location within the request signal space in which the
symbol-level request is transmitted. Thus, the existence of a
symbol-level request in the request signal space, as well as the
location where the symbol-level request exists in the request
signal space, can convey important information that is used to
facilitate the assignment of transmission opportunities within the
shared communication medium.
Request Signal Space and Scheduled Transmission Signal Space
[0059] According to various embodiments of the invention, the
shared communication medium utilized by access nodes 104, 106, 108,
and 110 may be organized into a request signal space and a
scheduled transmission signal space. Just as an example, the shared
communication medium may be implemented as a satellite
"return-link" that allows signals to be sent from access nodes 104,
106, 108, and 110 to scheduler node 102.
[0060] The request signal space may be used by access nodes 104,
106, 108, and 110 to send requests--e.g., symbol-level requests--to
request opportunities for the scheduled transmission of data.
Specifically, the request signal space may be organized into a
plurality of request segments. Each request segment generally
refers to a portion of request signal space that may be used for
sending a request.
[0061] The scheduled transmission signal space may be used by
access nodes 104, 106, 108, and 110 to transmit data, once requests
for transmission have been granted. The scheduled transmission
signal space may be organized into a plurality of scheduled
transmission segments. Each scheduled transmission segment
generally refers to a portion of scheduled transmission signal
space that may be used for sending a data transmission.
[0062] In accordance with the invention, the request signal space,
as well as the scheduled transmission signal space, may be
organized based on various multiplexing techniques. Thus, the
plurality of request segments in the request signal space may
represent allotments defined based on one or more types of
multiplexing techniques applied to the request signal space. As
mentioned previously, these may include time-division multiplexing,
frequency-division multiplexing, wavelet-division multiplexing,
code-division multiplexing, and/or other multiplexing techniques.
Similarly, the plurality of scheduled transmission segments in the
scheduled transmission space may represent allotments defined based
on one or more types of multiplexing techniques applied to the
scheduled transmission signal space.
[0063] As such, each request segment may have a different
"location" within the request signal space. For example, if a
request signal space is organized according to a time-division
multiplexing technique, each request segment may comprise a
different time slot in the request signal space. Here, each
particular request segment is said to correspond to a different
location (in time) in the request signal space. The same concept
can be applied to a request signal space organized according to a
frequency-division multiplexing technique. In such a case, each
request segment may comprise a particular frequency sub-channel and
be said to correspond to a different location (in frequency) in the
request signal space. The same concept can be applied to a request
signal space organized according to a code-division multiplexing
technique. In such a case, each request segment may comprise a
particular code word and be said to correspond to a different
location (in code space) in the request signal space. Similarly,
the concept can be applied to a request signal space organized
according to a combination of different multiplexing techniques,
such as a combination of time-division multiplexing and
frequency-division multiplexing techniques. In this particular
example, each request segment may comprises a particular time slot
in a particular frequency sub-channel and be said to correspond to
a different location (in time and frequency) in the request signal
space.
[0064] Also, the separation between the request signal space and
the scheduled transmission signal space may be based on different
multiplexing techniques. In one embodiment, time-division
multiplexing is employed. For example, the request signal space and
the scheduled transmission signal space may be defined over
different time slots and a common frequency range. In another
embodiment, frequency-division multiplexing is employed. For
example, the request signal space and the scheduled transmission
signal space may be defined over a common time duration and
different frequency ranges. In yet another embodiment,
code-division multiplexing is employed. For example, the request
signal space and the scheduled transmission signal space may be
defined over a common time duration and a common frequency range,
but using different code words.
Feedback Signal Space
[0065] In addition, a feedback signal space may be utilized for
sending the assignment messages from scheduler node 102 to access
nodes 104, 106, 108, and 110. In the present embodiment of the
invention, the feedback space is not a part of the shared
communication medium. Continuing with a satellite system example,
the feedback signal space may be implemented as a satellite
"forward-link" that allows signals to be sent from scheduler node
102 to access nodes 104, 106, 108, and 110. This satellite
"forward-link" may be separate from the "return-link" mentioned
previously.
[0066] The present invention broadly covers different combinations
of multiplexing techniques as applied to the request signal space
and/or the scheduled transmission signal space. In figures
discussed below, a number of examples of such multiplexing
combinations are presented. The various combinations of
multiplexing techniques described below are presented for
illustrative purposes and are not intended to restrict the scope of
the invention. In some examples, a feedback signal space is also
explicitly shown along with the request signal space and the
scheduled transmission signal space.
[0067] In the figures below, only a representative portion of the
relevant signal space is shown. For example, if four frames of
signals are shown, it should be understood that more frames may be
used even though they are not explicitly illustrated. Also, the
particular proportions of the various signal space designs are
provided as mere examples.
TDM Request Signal Space and Scheduled Transmission Signal Space
Partitioning with TDM Request Segments and Scheduled Transmission
Segments
[0068] FIG. 5 is an illustrative signal diagram showing time
division multiplexing (TDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with TDM
request segments and scheduled transmission segments, according to
an embodiment of the invention. The figure shows a representation
of shared communication medium 200 that includes a request signal
space and a scheduled transmission signal space. Separately, the
figure shows a feedback signal space 250.
[0069] In this particular embodiment of the invention, the shared
communication medium 200 is organized as one continuous sequence of
TDM time slots. For example, the shared communication medium 200
may comprise a particular frequency channel. Each TDM time slot
occupies the entire bandwidth of the frequency channel, but only
for a specific time duration. Here, the TDM time slots are shown as
being organized into "frames," such as Frame 0, Frame 1, Frame 2,
and Frame 3. For ease of illustration, FIG. 5 presents the TDM time
slots in multiple columns, instead of one continuous column.
Nevertheless, it should be understood that the TDM time slots
represent a single sequence of time slots that are sequentially
transmitted.
[0070] For example, FIG. 5 shows that Frame 0 includes 512 TDM time
slots. These 512 TDM time slots are shown as being arranged in a
rectangular grid having 16 columns (Column 0 through Column 15) and
32 rows (Row 0 through Row 31). The sequence of TDM time slots is
arranged in time as follows. Slots 0 through 31 of Column 1,
followed by Slots 0 through 31 of Column 2, followed by Slots 0
through 31 of Column 3, and so on. In this manner, the entire
sequence of 512 time slots in Frame 0 is arranged sequentially in
time. Frame 1 is structured similarly and follows Frame 0. That is,
the last time slot of Frame 0 is followed by the first slot of
Frame 1. Frame 2 is structured similarly and follows Frame 1. Frame
3 is structure similarly and follows Frame 2, and so on. Thus, the
entire sequence of TDM time slots contained in all the frames,
including Frames 0, 1, 2, and 3, is arranged sequentially in
time.
[0071] In the particular grid representation shown in FIG. 5, time
can be seen as proceeding down each column of TDM time slots. Hence
the direction down each column, across multiple rows, is labeled as
"Fast Row Time." Only after proceeding through all the TDM time
slots in a column can the next column be started. Thus, it takes
longer to proceed from one column to the next. Hence the direction
across multiple columns is labeled as "Slow Column Time."
[0072] In FIG. 5, the request signal space and scheduled
transmission signal space are defined on the basis of these TDM
time slots. Thus, in this example, the request signal space and the
scheduled transmission signal space are separated using TDM
multiplexing. Here, each frame includes a number of request
segments and a single scheduled transmission segment. In Frame 0,
for example, the first 32 time slots are considered 32 request
segments (Column 0). The next 480 time slots are considered one
scheduled transmission segment, made up of 480 symbols (Columns 1
through 15). Other frames such as Frames 0, 1, 2, and 3 are
structured in a similar manner.
[0073] FIG. 5 shows the following types of signals allotments: (1)
Unoccupied Request Slot, (2) Occupied Request Slot, and (3)
Scheduled Transmission Data Symbol. In this example, only some of
the available request segments are occupied by actual requests sent
from one or more access nodes. Thus, some request segments are
shown as unoccupied request slots, and others are shown as occupied
request slots. Each scheduled transmission segment is shown as
including a number of symbols, referred to as schedule transmission
data symbols.
[0074] According to an embodiment of the invention, when an access
node such as nodes 104, 106, 108, and 110 in FIG. 1 needs to
request a scheduled transmission, it sends out a request in one of
the request segments. Here, it is assumed that a TDM system is
implemented in which all of the nodes are time-synchronized, such
that every node has the capability to send signals in the
appropriate time slots. Of course in practice, signals sent from
various nodes may not arrive in their respective time slots with
perfect timing accuracy. The TDM system may be designed to handle
such imperfections, up to certain tolerances.
[0075] In one example, the request signals shown in FIG. 5 may be
sent as follows. Node 104 may send a request in Slot 7 of the
request signal space (Column 0) in Frame 0. Node 106 may send a
request in Slot 2 of the request signal space (Column 0) in Frame
1. Node 108 may send a request in Slot 26 of the request signal
space (Column 0) in Frame 1. Finally, node 110 may send a request
in Slot 18 of the request signal space (Column 0) in Frame 3.
Again, access nodes 104, 106, 108, and 110 are shown in FIG. 1. Of
course, an access node may also send multiple requests, sometimes
in the same frame. Thus, in an alternative example, all four of the
requests shown in FIG. 5 may be sent from node 104. That is, node
104 may send the request in Slot 7 of the request signal space
(Column 0) in Frame 0, the requests in Slot 2 and Slot 26 of the
request signal space (Column 0) in Frame 1, and the request in Slot
18 of the request signal space (Column 0) in Frame 3.
[0076] According to an embodiment of the invention, scheduler node
102 shown in FIG. 1 receives the requests and makes assignments to
assign each request to a scheduled transmission segment. Thus, in
response to the requests, scheduler 102 send out assignment
messages in a feedback signal space 250. The assignment messages
are broadcast to access nodes 104, 106, 108, and 110, to inform the
access nodes of the assignments made, so that each access node may
correctly send data in the assigned scheduled transmission
segment(s).
[0077] FIG. 5 depicts an assignment message under a "robust FIFO
schedule mode," in accordance with one embodiment of the invention.
In this mode, each assignment message explicitly includes a pair of
data: (1) an identifier for the request and (2) an identifier for
the scheduled transmission segment associated with the request. In
other words, the pairing of a request to an associated scheduled
transmission segment is directly stated in the assignment message.
For example, as shown in FIG. 5, the first assignment message
includes the pair of data "REQ 0:7, SCH 13." This indicates that
the request sent in the request segment known as "REQ 0:7" (the
request segment located at Frame 0, Slot 7) has been assigned the
scheduled transmission segment known as "SCH 13" (the scheduled
transmission segment located in Frame 13). The rest of the
assignment messages follow a similar format. The second assignment
message includes the pair of data "REQ 1:2, SCH 14." The third
assignment message includes the pair of data "REQ 1:26, SCH 15."
The fourth assignment message includes the pair of data "REQ 3:18,
SCH 16."
[0078] The entire request and assignment process takes place in an
anonymous manner with respect to the identity of the access nodes,
according to an embodiment of the invention. Thus, a symbol-level
request sent from an access node does not explicitly identify the
access node. For example, assume that access node 104 sent the
symbol-level request in "REQ 0:7" (Slot 7 of the request signal
space of Frame 0). This symbol-level request is merely a symbol
transmitted at a particular location within the request symbol
space. The symbol-level request does not explicitly identify the
access node 104. Similarly, the corresponding assignment message
"REQ 0:7, SCH 13" broadcast from scheduler node 102 does not
explicitly identify access node 104 as the intended recipient of
the assignment message. Instead, the assignment message merely
announces that the symbol-level request sent in the "REQ 0:7" slot
has been assigned to the scheduled transmission segment "SCH 13."
All of the access nodes 104, 106, 108, and 110 receive the
broadcast assignment message. However, only access node 104 accepts
the assignment and proceeds to send a data transmission in the
scheduled transmission segment identified by the assignment. This
is possible because each access node keeps track of the location(s)
of the symbol-level request(s) it has sent. Access node 104
recognizes the request "REQ 0:7" identified in the assignment as
one of its own and thus accepts the assignment. The other access
nodes 106, 108, and 110 do not recognize the request "REQ 0:7"
identified in the assignment as one of their own and thus do not
accept the assignment.
[0079] In FIG. 5, the feedback signal space 250 is labeled as
"Delayed Feedback Grant Channel." In this particular embodiment of
the invention, an assignment message sent in feedback signal space
250 may be delayed in the sense that it may not be broadcast until
some time after (perhaps multiple frame after) the initial request
is made.
Two-Service TDM Request and Scheduled Transmission
[0080] FIG. 6 is an illustrative signal diagram showing a
two-service TDM request and scheduled transmission scheme, in
accordance with an embodiment of the invention. The system shown in
FIG. 6 is similar to the system shown in FIG. 5 in many respects.
FIG. 6 shows a representation of a shared communication medium 300
that includes a request signal space and a scheduled transmission
signal space. TDM is utilized to partition the request signal space
and the scheduled transmission signal space. TDM request segments
and scheduled transmission segments are also shown. Separately,
FIG. 6 shows a feedback signal space 350.
[0081] Unlike FIG. 5, FIG. 6 introduces a two-service methodology.
The distinction between different services may be based on one or
more factors, such as the length of scheduled transmission, code
rate, modulation method, and/or others. In the example shown in
FIG. 6, the distinction between the two services is based on length
of the scheduled transmission. Thus, an access node can either
request a short scheduled transmission of data (1.sup.st service)
or request a long scheduled transmission of data (2.sup.nd
service). Here, a short scheduled transmission of data is also
referred to as a "short burst," and a long scheduled transmission
of data is also referred to as a "long burst."
[0082] According to an embodiment of the invention, the request
segments are organized into two categories, one for the first
service and the other for the second service. Similarly, the
scheduled transmission segments are organized into two categories,
short scheduled transmission segments for the first service and
long scheduled transmission segments for the second service. An
access node such as access nodes 104, 106, 108, and 110 chooses the
service desired by sending a request in a request segment of the
appropriate category. To request a short scheduled transmission of
data (1.sup.st service), an access node simply sends a request in
one of the 1.sup.st service request segments. To request a long
scheduled transmission of data (2.sup.nd service), the access node
simply sends a request in one of the 2.sup.nd service request
segments.
[0083] FIG. 6 shows the following types of signals allotments: (1)
Empty Short Burst Request Slot, (2) Empty Long Burst Request Slot,
(3) Occupied Request Slot, and (4) Scheduled Transmission Data
Symbol. The request segments comprise short burst request slots and
long burst request slots. As shown in this example, only some of
the available request segments are occupied by actual requests sent
from one or more access nodes. Thus, some request segments are
shown as unoccupied request slots, and others are shown as occupied
request slots. Each scheduled transmission segment is shown as
including a number of symbols, referred to as schedule transmission
data symbols.
[0084] In accordance with an embodiment of the invention, the data
frame structure may be used to organize the two categories of
request segments and scheduled transmission segments. In this
figure, even numbered frames such as Frames 0, 2, 4, etc., contain
request segments and scheduled transmission segments for short
scheduled transmissions of data (1.sup.st service). Odd numbered
frames such as Frames 1, 3, 5, etc., contain request segments and
scheduled transmission segments for long scheduled transmission of
data (2.sup.nd service).
[0085] FIG. 6 presents a number of illustrative requests for the
two service types, as sent from one or more access nodes such as
104, 106, 108, and 110. A short burst request is sent in Slot 7 of
the request signal space in Frame 0. Two long burst requests are
sent in Slots 2 and 26 of the request signal space in Frame 1. A
short burst request is sent in Slot 22 of the request signal space
in Frame 2. A long burst request is sent in Slot 8 of the request
signal space in Frame 3. Finally, a short burst request is sent in
Slot 19 of the request signal space in Frame 4.
[0086] In response, scheduler node 102 broadcasts assignment
messages in feedback signal space 350. As shown in FIG. 6, the
first assignment message "REQ 0:07, SHORT 12" indicates that the
request sent in Slot 7 of the request signal space in Frame 0 has
been assigned to the short scheduled transmission segment in Frame
12. The second assignment message "REQ 1:02, LONG 17" indicates
that the request sent in Slot 2 of the request signal space in
Frame 1 has been assigned to the long scheduled transmission
segment in Frame 17. The third assignment message "REQ 1:26, LONG
19" indicates that the request sent in Slot 26 of the request
signal space in Frame 1 has been assigned to the long scheduled
transmission segment in Frame 19. The fourth assignment message
"REQ 2:22, SHORT 14" indicates that the request sent in Slot 22 of
the request signal space in Frame 1 has been assigned to the short
scheduled transmission segment in Frame 14. The fifth assignment
message "REQ 3:08, LONG 21" indicates that the request sent in Slot
8 of the request signal space in Frame 3 has been assigned to the
long scheduled transmission segment in Frame 21. The fifth
assignment message "REQ 4:19, SHORT 16" indicates that the request
sent in Slot 19 of the request signal space in Frame 4 has been
assigned to the short scheduled transmission segment in Frame
16.
[0087] The assignment messages are broadcast to access nodes 104,
106, 108, and 110, to inform the access nodes of the assignments
made, so that each access node may correctly send data in the
assigned scheduled transmission segment(s). While a two-service
example is presented in FIG. 6, systems having more than two types
of service are within the scope of the present invention. Such
systems can be implemented, for example, by adopting a signal space
design similar to that shown in FIG. 6 and scaling up the number of
categories of request segments and scheduled transmission
segments.
OFDM/TDM Request and Scheduled Transmission
[0088] FIG. 7 is an illustrative signal diagram showing orthogonal
frequency division multiplexing (OFDM) as utilized to partition the
request signal space and the scheduled transmission signal space,
with OFDM/TDM request segments and scheduled transmission segments,
according to an embodiment of the invention. The figure shows a
representation of a shared communication medium 400 based on an
OFDM structure that includes a request signal space and a scheduled
transmission signal space. Separately, the figure shows a feedback
signal space 450. In this particular embodiment of the invention,
the shared communication medium 400 is organized into 34 different
OFDM frequency channels, labeled as subcarriers -1 through -17 and
1 through 17. Each of these 34 different OFDM frequency channels is
further organized into TDM time slots. The DC channel, labeled as
subcarrier 0, is left unused.
[0089] According to the present embodiment of the invention, OFDM
frequency channels provide a natural separation between the request
signal space and the scheduled transmission signal space. As shown
in FIG. 7, the request signal space comprises OFDM frequency
channels -1 and 1. The scheduled transmission signal space
comprises OFDM frequency channels -2 through -17 and 2 through
17.
[0090] In this embodiment, each request segment comprises a TDM
time slot in one of the OFDM frequency channels -1 and 1. Each
scheduled transmission segment comprises a block of 512 data
symbols, spanning 16 OFDM frequency channels and 32 TDM time slots.
FIG. 7 shows four such scheduled transmission segments. The first
scheduled transmission segment spans OFDM frequency channels -2
through -17 and TDM time slots 0 through 31. The second scheduled
transmission segment spans OFDM frequency channels 2 through 17 and
TDM time slots 0 through 31. The third scheduled transmission
segment spans OFDM frequency channels -2 through -17 and TDM time
slots 32 through 63. The fourth scheduled transmission segment
spans OFDM frequency channels 2 through 17 and TDM time slots 32
through 63. Thus, FIG. 7 presents request segments based on OFDM
and TDM, as well as scheduled transmission segments based on OFDM
and TDM.
[0091] FIG. 7 shows the following types of signals allotments: (1)
Unoccupied Request Slot, (2) Occupied Request Slot, and (3)
Scheduled Transmission Data Burst. In this example, only some of
the available request segments are occupied by actual requests sent
from one or more access nodes. Thus, some request segments are
shown as unoccupied request slots, and others are shown as occupied
request slots. Each scheduled transmission segment comprises a
block of 512 data symbols referred to here as a data burst.
[0092] FIG. 7 presents a number of illustrative requests, as sent
from one or more access nodes such as 104, 106, 108, and 110. A
request is sent in Slot 8 in OFDM frequency channel -1. Another
request is sent in Slot 18 in OFDM frequency channel 1. Another
request is sent in Slot 22 in OFDM frequency channel 1. Yet another
request is sent in Slot 57 in OFDM frequency channel 1.
[0093] In response, scheduler node 102 broadcasts assignment
messages in feedback signal space 450. As shown in FIG. 7, the
first assignment message "REQ -8, SCH +21" indicates that the
request sent in Slot 8 in OFDM frequency channel -1 has been
assigned to the scheduled transmission segment +21. In this
example, a scheduled transmission segment is identified by a sign
(positive or negative) and a sequence number. For instance, the
scheduled transmission segment +21 has a positive sign (+), which
indicates that this scheduled transmission segment is located on
the positive side of the OFDM frequency channels (spanning the 16
positive OFDM frequency channels). The sequence number 21 indicates
that this scheduled transmission segment is the 21.sup.st one in a
sequence of scheduled transmissions.
[0094] The rest of the assignment messages can be interpreted in a
similar way. The second assignment message "REQ+18, SCH-22"
indicates that the request segment in Slot 18 in OFDM frequency
channel 1 has been assigned to the scheduled transmission segment
-22. The third assignment message "REQ+22, SCH+22" indicates that
the request segment in Slot 22 in OFDM frequency channel 1 has been
assigned to the schedule transmission segment +22. The fourth
assignment message "REQ+57, SCH-23" indicates that the request
segment in Slot 57 in OFDM frequency channel 1 has been assigned to
the scheduled transmission segment -23.
[0095] The assignment messages are broadcast to access nodes 104,
106, 108, and 110, to inform the access nodes of the assignments
made, so that each access node may correctly send data in the
assigned scheduled transmission segment(s).
[0096] FIG. 8 is an illustrative signal diagram showing another
example of orthogonal frequency division multiplexing (OFDM) as
utilized to partition the request signal space and the scheduled
transmission signal space, with OFDM/TDM request segments and
scheduled transmission segments, according to an embodiment of the
invention. The figure shows a representation of a shared
communication medium 500 based on an OFDM structure that includes a
request signal space and a scheduled transmission signal space.
Separately, the figure shows a feedback signal space 550. The
shared communication medium 500 is once again organized into 34
different OFDM frequency channels, labeled as subcarriers -1
through -17 and 1 through 17. Each of these 34 different OFDM
frequency channels is further organized into TDM time slots. The DC
channel, labeled as subcarrier 0, is left unused.
[0097] This example demonstrates that the request signal space and
scheduled transmission signal space can be defined in a different
way. Again, OFDM frequency channels are used to provide the
separation between request signal space and scheduled transmission
signal space. However, a different pair of OFDM frequency channels
is used here to implement the request signal space. Here, the
request signal space comprises OFDM frequency channels -17 and 17.
The scheduled transmission signal space comprises OFDM frequency
channels -1 through -16 and 1 through 16.
[0098] This example also demonstrates that the scheduled
transmission segments can be defined as utilizing a different
allocation of OFDM frequency channels and TDM time slots. Here,
each scheduled transmission segment still comprises a block of 512
data symbols. However, the 512 data symbols span 32 OFDM frequency
channels and 16 TDM time slots. FIG. 8 shows four such scheduled
transmission segments. The first scheduled transmission segment
spans OFDM frequency channels -2 through -17 and 2 through 17 and
TDM time slots 0 through 15. The second scheduled transmission
segment spans OFDM frequency channels -2 through -17 and 2 through
17 and TDM time slots 16 through 31. The third scheduled
transmission segment spans OFDM frequency channels -2 through -17
and 2 through 17 and TDM time slots 32 through 47. The fourth
scheduled transmission segment spans OFDM frequency channels -2
through -17 and 2 through 17 and TDM time slots 48 through 63.
[0099] FIG. 8 shows the following types of signals allotments: (1)
Unoccupied Request Slot, (2) Occupied Request Slot, and (3)
Scheduled Transmission Data Burst. In this example, only some of
the available request segments are occupied by actual requests sent
from one or more access nodes. Thus, some request segments are
shown as unoccupied request slots, and others are shown as occupied
request slots. Each scheduled transmission segment comprises a
block of 512 data symbols referred to here as a data burst.
[0100] FIG. 8 presents a number of illustrative requests, as sent
from one or more access nodes such as 104, 106, 108, and 110. A
request is sent in Slot 4 in OFDM frequency channel -17. Another
request is sent in Slot 8 in OFDM frequency channel -17. Another
request is sent in Slot 22 in OFDM frequency channel 17. Another
request is sent in Slot 33 in OFDM frequency channel -17. Yet
another request is sent in Slot 57 in OFDM frequency channel
17.
[0101] In response, scheduler node 102 broadcasts assignment
messages in feedback signal space 550. As shown in FIG. 8, the
first assignment message "REQ -4, SCH 11" indicates that the
request sent in Slot 4 in OFDM frequency channel -17 has been
assigned to the scheduled transmission segment 11. In this example,
a scheduled transmission segment is identified by a sequence
number. No sign is used because each scheduled transmission segment
spans all OFDM frequency channels, including all positive and
negative frequency channels. The sequence number 11 indicates that
this scheduled transmission segment is the 11.sup.th one in a
sequence of scheduled transmissions.
[0102] The rest of the assignment messages can be interpreted in a
similar way. The second assignment message "REQ -8, SCH 12"
indicates that the request segment in Slot 8 in OFDM frequency
channel -17 has been assigned to the scheduled transmission segment
12. The third assignment message "REQ +22, SCH 13" indicates that
the request segment in Slot 22 in OFDM frequency channel 17 has
been assigned to the schedule transmission segment 13. The fourth
assignment message "REQ -33, SCH 14" indicates that the request
segment in Slot 33 in OFDM frequency channel -17 has been assigned
to the scheduled transmission segment 14. The fifth assignment
message "REQ +57, SCH 15" indicates that the request segment in
Slot 57 in OFDM frequency channel 17 has been assigned to the
scheduled transmission segment 15. The assignment messages are
broadcast to access nodes 104, 106, 108, and 110, to inform the
access nodes of the assignments made, so that each access node may
correctly send data in the assigned scheduled transmission
segment(s).
FDM Request and Scheduled Transmission
[0103] FIG. 9 is an illustrative signal diagram showing frequency
division multiplexing (FDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with
FDM/TDM request segments and scheduled transmission segments,
according to an embodiment of the invention. The figure shows a
representation of a shared communication medium 600 based on an FDM
structure that includes a request signal space and a scheduled
transmission signal space. The request signal space comprises a
narrow FDM frequency channel, labeled as REQ Carrier. The scheduled
transmission signal space comprises two wide FDM frequency
channels, labeled as SCH Carrier 1 and SCH Carrier 2. A feedback
signal space is not explicitly shown in this figure but may also be
implemented in a manner similar to that described with respect to
previous figures.
[0104] This example demonstrates that the request signal space and
the scheduled transmission signal space may have very different
symbol structures. In the request signal space, a symbol is
transmitted over the narrow REQ Carrier channel over a longer time
duration. These are labeled as REQ Symbol Slots 0, 1, 2, 3, etc. in
the figure. By contrast, in the schedule transmission signal space,
a symbol is transmitted over one of the two wide SCH Carrier
channels over a shorter time duration. These are labeled as Data
Burst Symbol 0, 1, 2, 3, . . . , 63, etc. in the figure. Despite
this difference in symbol structures, a symbol transmitted in the
request signal space may have the same time-bandwidth product as a
symbol transmitted in the scheduled transmission signal space,
according to an embodiment of the present invention. Thus, FIG. 9
presents request segments based on FDM and TDM, as well as
scheduled transmission segments based on FDM and TDM.
[0105] FIG. 9 shows the following types of signals allotments: (1)
Unoccupied Request Slot and (2) Occupied Request Slot. In this
example, only some of the available request segments are occupied
by actual requests sent from one or more access nodes. Thus, some
request segments are shown as unoccupied request slots, and others
are shown as occupied request slots. FIG. 9 also shows scheduled
transmission segments. Each scheduled transmission segment
comprises a block of N data symbols referred to here as a data
burst. Only a portion of the data burst is shown in the figure.
[0106] FIG. 9 presents an illustrative request as sent from one or
more access nodes such as 104, 106, 108, and 110. The request shown
is sent in Slot 1 of the REQ Carrier channel. In response,
scheduler node 102 broadcasts assignment messages in feedback
signal space (not shown). The assignment messages are broadcast to
access nodes 104, 106, 108, and 110, to inform the access nodes of
the assignments made, so that each access node may correctly send
data in the assigned scheduled transmission segment(s).
[0107] Also shown in FIG. 9 are guard zones, specifically frequency
guard bands, positioned between various carriers. A first frequency
guard band is positioned between the SCH Carrier 1 channel and REQ
Carrier channel. A second frequency guard band is positioned
between the REQ Carrier channel and the SCH Carrier 2 channel. The
use of these guard bands can improve reception and processing of a
signal on a particular carrier by providing separation and reduced
interference from neighboring carriers.
TDM Request Signal Space and Scheduled Transmission Signal Space
Partitioning with Synchronous CDM Request Segments
[0108] FIG. 10 is an illustrative signal diagram showing time
division multiplexing (TDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with
synchronous CDM request segments, according to an embodiment of the
invention. The figure shows a representation of a shared
communication medium 700 based on a TDM structure that includes a
request signal space and a scheduled transmission signal space. A
feedback signal space is not explicitly shown in this figure but
may also be implemented in a manner similar to that described with
respect to previous figures.
[0109] The structure shown in FIG. 10 is based on sequentially
ordered frames. Seventeen such frames are shown in this figure,
labeled by frame indices 0 through 16. Additional frames may
follow. Each frame has a total length of 456 symbols. This total
length is divided between a scheduled transmission signal space
portion having a length of 424 symbols and a request signal space
portion having a length of 32 symbols.
[0110] For ease of illustration, the numerous symbols are not
individually shown in this figure. Instead, boxes representing
multiple symbols are shown. In the transmission signal space, each
short box represents 8 scheduled transmission symbols. In the
request signal space, each long box represents a 32-chip CDMA
request interval. Although the signal segments representing
individual chips of any particular CDMA code may be similar in
design to the signal segments representing the scheduled
transmission symbols, the chips of any particular code are linked
in a particular code pattern (e.g., a 32-chip pattern), whereas the
scheduled transmission symbols may be individually modulated. As
shown, FIG. 10 presents scheduled transmission segments based on
TDM and request segments based on CDM.
[0111] More specifically, each 456-symbol frame supports 1
scheduled transmission segment and 32 request segments. The 1
scheduled transmission segment comprises the first 424 symbols of
the frame. The 32 request segments comprise the 32 possible code
words that may be transmitted in the remaining portion of the
frame. In other words, the remaining portion of the frame is code
division multiplexed and organized as a 32-chip request
interval.
[0112] Here, a 32-chip Walsh CDMA code is used. In this code space,
there exist 32 different possible code words, each having a length
of 32 chips. Indices 0 through 31 are used to identify the 32
different possible code words. FIG. 10 shows the chip-level detail
of the 32 code words. Other types and lengths of code may be used
in accordance with the invention.
[0113] One or more of the access nodes 104, 106, 108, and 110 can
send one or more requests (each in the form of one of the 32
possible code words) in a particular request interval. This is
illustrated in FIG. 10. In the example shown, two requests are sent
in a request interval. The first request is a signal spread
according to code word 13. The second request is a signal spread
according to code word 22. Thus, code division multiplexing allows
the request interval to support 32 request segments, i.e., codes
slots. As shown in FIG. 10, two of these request segments are
occupied. The remaining thirty request segments are unoccupied.
[0114] In FIG. 10, it is assumed that a TDM system is implemented
in which all of the nodes are sufficiently time-synchronized, such
that no guard zone is needed to separate the scheduled transmission
symbols and the request intervals. Even though minor timing offsets
may exist between different CDM request signals, the requests are
still received and processed with acceptable performance. As such,
FIG. 10 refers to a system having "synchronous" CDM request
segments.
[0115] In response, scheduler node 102 broadcasts assignment
messages in feedback signal space (not shown). The assignment
messages are broadcast to access nodes 104, 106, 108, and 110, to
inform the access nodes of the assignments made, so that each
access node may correctly send data in the assigned scheduled
transmission segment(s).
[0116] Again, the entire request and assignment process takes place
in an anonymous manner with respect to the identity of the access
nodes. Thus, a symbol-level request sent from an access node does
not explicitly identify the access node. For example, assume that
access node 104 sends the symbol-level request comprising code word
13. This symbol-level request is merely a signal transmitted at a
particular code location within the request symbol space. The
symbol-level request does not explicitly identify the access node
104.
[0117] Similarly, the corresponding assignment message would not
explicitly identify access node 104 as the intended recipient of
the assignment message. Instead, the assignment message merely
announces that the symbol-level request corresponding to code word
13 in Frame 0 has been assigned to a particular scheduled
transmission segment. All of the access nodes 104, 106, 108, and
110 receive the broadcast assignment message. However, only access
node 104 accepts the assignment and proceeds to send a data
transmission in the scheduled transmission segment identified by
the assignment. This is possible because each access node keeps
track of the location(s) in code space of the symbol-level
request(s) it has sent in each frame. Access node 104 recognizes
the request identified in the assignment as one of its own and thus
accepts the assignment. The other access nodes 106, 108, and 110 do
not recognize the request identified in the assignment as one of
their own and thus do not accept the assignment.
TDM Request Signal Space and Scheduled Transmission Signal Space
Partitioning with Quasi-Synchronous CDM Request Segments
[0118] FIG. 11 is an illustrative signal diagram showing time
division multiplexing (TDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with
quasi-synchronous CDM request segments, according to an embodiment
of the invention. The figure shows a representation of a shared
communication medium 800 based on a TDM structure that includes a
request signal space and a scheduled transmission signal space. A
feedback signal space is not explicitly shown in this figure but
may also be implemented in a manner similar to that described with
respect to previous figures.
[0119] The structure shown in FIG. 11 is similar to that of FIG.
10, except that 8-symbol guard zones are inserted in each frame to
separate the scheduled transmission signal space and the request
signal space. The structure is again based on sequentially ordered
frames. Seventeen such frames are shown in this figure, labeled by
frame indices 0 through 16. Additional frames may follow. Each
frame has a total length of 472 symbols. This total length is
divided among four different portions of the frame: (1) a guard
zone having a length of 8 symbols, (2) a scheduled transmission
signal space portion having a length of 424 symbols, (3) another
guard zone having a length of 8 symbols, and (4) a request signal
space portion having a length of 32 code chips.
[0120] For ease of illustration, the numerous symbols are not
individually shown in this figure. Instead, boxes representing
multiple symbols are shown. In the transmission signal space, each
short box represents 8 scheduled transmission symbols. In the
request signal space, each long box represents a 32-chip CDMA
request interval. For the guard zones, each short box represents 8
guard symbols. Thus, FIG. 11 presents scheduled transmission
segments based on TDM and request segments based on CDM.
[0121] A 32-chip, low cross-correlation, quasi-synchronous CDMA
code is used in this embodiment. When two or more of such codes are
not synchronously aligned in time (e.g., they are mis-aligned by a
chip), the cross correlation between the codes remains relatively
low. Other types and lengths of code may be used in accordance with
the invention. One or more of the access nodes 104, 106, 108, and
110 can send one or more requests (each in the form of one of the
32 possible code words) in a particular request interval. In the
example shown, two requests are sent in a request interval. The
first request is a signal spread according to code word 13. The
second request is a signal spread according to code word 22. Thus,
code division multiplexing allows the request interval to support
32 request segments, i.e., codes slots. As shown in FIG. 11, two of
these request segments are occupied. The remaining thirty request
segments are not unoccupied.
[0122] In FIG. 11, it is assumed that a TDM system is implemented
in which the nodes are not sufficiently time-synchronized, such
that guard zones are needed to separate the scheduled transmission
symbols and the request intervals. Because of the imperfect time
synchronization, different requests received may be time-offset
from one another. As a result, energy from immediately adjacent
scheduled transmission symbols may interfere with and degrade the
proper reception and processing of the requests. By inserting guard
zones, the likelihood of such encroachment is reduced, thereby
allowing better performance when received requests, such as
multiple CDM requests received in a particular request interval,
are not precisely synchronized in time. As such, FIG. 11 refers to
a system having "quasi-synchronous" CDM request segments.
[0123] In response, scheduler node 102 broadcasts assignment
messages in feedback signal space (not shown). The assignment
messages are broadcast to access nodes 104, 106, 108, and 110, to
inform the access nodes of the assignments made, so that each
access node may correctly send data in the assigned scheduled
transmission segment(s).
TDM Request Signal Space and Scheduled Transmission Signal Space
Partitioning with Synchronous CDM Request Segments
[0124] FIG. 12 is an illustrative signal diagram showing code
division multiplexing (CDM) as utilized to partition the request
signal space and the scheduled transmission signal space, with CDM
request segments and CDM scheduled transmission segments, according
to an embodiment of the invention. The figure shows a
representation of a shared communication medium 900 based on a
concatenated CDM structure that utilizes an inner code and an outer
code to support a request signal space and a scheduled transmission
signal space. A feedback signal space is not explicitly shown in
this figure but may also be implemented in a manner similar to that
described with respect to previous figures.
[0125] The structure shown in FIG. 12 is based on sequentially
ordered frames. Two such frames are shown in this figure. Frame #1
comprises the block of signal space covering "Inner Walsh Code
Index" 0 through 31 and "Inner Walsh Code Spread Interval Index" 0
through 15. Frame #2 comprises a block of the signal space covering
"Inner Walsh Code Index" 0 through 31 and "Inner Walsh Code Spread
Interval Index" 16 through 31. The frames have similar formats. As
such, Frame #1 is described in detail below as an illustrative
example.
[0126] Frame #1 contains 16 time slots (corresponding to the 16
columns of Frame #1 shown in FIG. 12). Each time slot represents an
inner code interval of 32 chips. A detailed view of one such
32-chip inner code interval is shown at the bottom of FIG. 12.
Here, an "inner code" is employed to multiplex each time slot into
32 "inner code channels," one for each code word. The 32 "inner
code channels" are shown as the 32 rows of Frame #1 in FIG. 12.
Here, a 32-chip Walsh code having 32 different possible code words
is used.
[0127] Thus, instead of transmitting a single symbol in each time
slot, up to 32 different code symbols can be transmitted in each
time slot. Each code symbol comprises a particular 32-chip inner
code word, modulated by the data value that is to be sent. For
example, the 32-chip code word for a particular inner code channel
may be [0, 0, 0, . . . , 0]. Here, modulating this inner code word
with a data value of "0" results in a 32-chip code symbol of [0, 0,
0, . . . , 0]. Modulating the same inner code word with a data
value of "1" results in a 32-chip code symbol of [1, 1, 1, . . . ,
1]. As mentioned above, Frame #1 contains 16 time slots, and each
time slot supports 32 different code symbols (corresponding to 32
inner code channels). Thus, Frame #1 supports 512 code symbols.
[0128] The multiplexing provided by the "inner code channels" is
used to partition Frame #1 into a portion of request signal space
and a portion of scheduled transmission signal space. As shown in
FIG. 12, Frame #1 includes 32 inner code channels labeled as "Inner
Walsh Code Index" 0 through 32. The first two code channels, 0 and
1, are designated as the request signal space portion. The
remaining thirty code channels, 2 through 31, are designated as the
scheduled transmission portion. Thus, each frame supports a
scheduled transmission segment comprising 480 code symbols (30
inner code channels.times.16 time slots). In other words, 480 of
the 512 code symbols in Frame #1 are use as part of the scheduled
transmission segment.
[0129] The request signal space, comprising (1) a first "Outer
Request Code Spread Interval" inner code channel 0 and (2) a second
"Outer Request Code Spread Interval" in code channel 1, is further
multiplexed using an "outer" code. The outer code has a code length
of 16 chips and spans 16 time slots. That is, each chip of the
16-chip outer code spans an entire time slot. Here, a Walsh code
having 16 different possible code words and a length of 16 chips is
used. As a result, each "Outer Request Code Spread Interval" is
further multiplexed, into 16 "outer code channels." Each of these
outer code channels represents a unique request segment in the
request signal space. Frame #1 has two such "Outer Request Code
Spread Intervals." Accordingly, Frame #1 supports a total of 32
different request segments.
[0130] Given this structure, each request signal is transmitted
using a certaini 6-chip outer code word and a certain 32-chip inner
code word. Furthermore, each request signal spans 16 time slots. In
each time slot, a particular "chip" of the 16-chip outer code word
modulates an entire 32-chip inner code word. Just as a simple
example, the 32-chip inner code word may be [0, 1, 0, 1, . . . , 0,
1]. Here, modulating this inner code word with an outer code word
"chip" having a value of "0" produces a request signal portion [0,
1, 0, 1, . . . , 0, 1]. Modulating the same inner code word with an
outer code word "chip" having a value of "1" produces a request
signal portion [1, 0, 1, 0, . . . , 1, 0].
[0131] One or more of the access nodes 104, 106, 108, and 110 can
send one or more requests. Each request can be sent in one of the
32 different request segments in a frame. In the example depicted
in FIG. 12, a request in Frame #2 is sent in the request segment
shown as inner code channel 0 and outer code channel 13.
[0132] In response, scheduler node 102 broadcasts assignment
messages in feedback signal space (not shown). The assignment
messages are broadcast to access nodes 104, 106, 108, and 110, to
inform the access nodes of the assignments made, so that each
access node may correctly sesnd data in the assigned scheduled
transmission segment(s).
Orthogonal and Non-Orthogonal Request Signal Designs
[0133] According to an embodiment of the invention, the request
segments may be positioned such that adjacent request segments may
represent orthogonal portions of the request signal space. Adjacent
request segments may refer to request segments that are located
next to one another in time, frequency, and/or code space. Here,
orthogonal portions of a signal space refer to allotments within
the signal space that are designed to be separable from one
another, at least under ideal conditions. For request segments
defined based on frequency division multiplexing, such orthogonal
requests may be achieved by implementing adjacent request segments
as non-overlapping frequency channels. For request segments defined
based on time division multiplexing, such orthogonal requests may
be achieved by implementing adjacent request segments as
non-overlapping time slots. For request segments defined based on
code division multiplexing, such orthogonal requests may be
achieved by implementing adjacent request segments using orthogonal
codes, such as Walsh codes as described in previous sections, Walsh
codes combined with a common pseudo noise (PN) sequence using
modulo 2 addition (Walsh codes with common PN sequence overlay),
Gold codes, and others.
[0134] According to an embodiment of the invention, the request
segments may be positioned such that adjacent request segments may
represent non-orthogonal portions of the request signal space.
Here, non-orthogonal portions of a signal space refer to allotments
within the signal space that are designed to be only partially
separable from one another. For request segments defined based on
frequency division multiplexing, such non-orthogonal requests may
be achieved by implementing adjacent request segments as
partially-overlapping frequency channels. For request segments
defined based on time division multiplexing, such non-orthogonal
requests may be achieved by implementing adjacent request segments
as partially-overlapping time slots. For request segments defined
based on code division multiplexing, such orthogonal requests may
be achieved by implementing adjacent request segments using
non-orthogonal codes.
[0135] A disadvantage associated with use of non-orthogonal request
segments is the reduction in reception performance. When
partially-overlapping frequency channels, partially-overlapping
time slots, and/or non-orthogonal codes are used, the reception of
particular request segment might capture some energy from adjacent
request segment(s), thus reducing reception performance. On the
other hand, an advantage associated with the use of non-orthogonal
request segments is that it allows for a more densely packed
request signal space. That is, more request segments can be packed
into a given request signal space.
[0136] According to various embodiments of the invention, the
extent of non-orthogonality is selected for a particular balance of
such advantages and disadvantages. In one example, non-orthogonal,
time-division multiplexed request segments may be implemented as 1
msec time slots, spaced 1/2 msec apart. That is, each time slot has
a length of 1 msec. However, the time slots are arranged to overlap
one another, such that a new time slot starts every 1/2 msec. Thus,
adjacent time slots have a 1/2 msec overlap region. This particular
balance is presented as one example and can be adjusted. The amount
of overlap can be increased (e.g., to greater than 1/2 msec), which
achieves an even more densely packed request signal space but
further degrades reception performance. Alternatively, the amount
of overlap can be decreased (e.g., to less than 1/2 msec), which
achieves a less densely packed request signal space but improves
reception performance. The level of non-orthogonality can be
similarly adjusted for request segments based on frequency division
multiplexing (by adjusting overlapping frequency channels) and code
division multiplexing (by selecting different non-orthogonal
codes).
[0137] Furthermore, different combinations of multiplexing type and
non-orthogonality technique can be employed. For example, request
segments may be based on a code division multiplexing employing
non-orthgonal codes and overlapping time slots. This may be
implemented, for instance, using non-orthogonal codes defined over
32 msec code intervals (time slots) spaced 16 msec apart. Thus,
adjacent code intervals have a 16 msec overlap region. In such a
case, non-orthogonality is attributed to the use of both
non-orthogonal codes and overlapping time slots. As another
example, request segments may be based on code division
multiplexing employing non-orthogonal codes and a common code
interval. In such a case, non-orthogonality is attributed only to
the use of non-orthogonal codes.
Robust FIFO Scheduling and Efficient FIFO Scheduling
[0138] FIG. 5 and other figures depict an assignment message under
a "robust FIFO schedule mode," in accordance with one embodiment of
the invention. As mentioned previously, in this mode, each
assignment message explicitly includes a pair of data: (1) an
identifier for the request and (2) an identifier for the scheduled
transmission segment associated with the request. In other words,
the pairing of a request to an associated scheduled transmission
segment is directly stated in the assignment message. For example,
as shown in FIG. 5, the first assignment message includes the pair
of data "REQ 0:7, SCH 13." This indicates that the request sent in
the request segment known as "REQ 0:7" (the request segment located
at Frame 0, Slot 7) has been assigned the scheduled transmission
segment known as "SCH 13" (the scheduled transmission segment
located in Frame 13).
[0139] Processing of a message under the "robust FIFO schedule
mode" by an access node such as 104, 106, 108, and 110 is described
below, according to one embodiment. Here each access node maintains
a local data transmission queue based on assignment messages
received at the first node. The local data transmission queue may
be a first-in-first-out (FIFO) list of all symbol-level-requests
from all access nodes that have been assigned to scheduled
transmission segments, arranged in the order of transmission. Each
entry in the queue may contain not only an identifier for a
symbol-level request, but also an identifier for the corresponding
scheduled transmission segment. The local data transmission queue
is synchronized to the timing of the rest of the system. Thus, as
each transmission occurs or is about to occur, the access node
examines the corresponding entry in the local data transmission
queue. If the access node recognizes the entry as being associated
with a symbol-level request sent from the access node, the access
node knows that its turn has come and sends its pending data
transmission. If the access node does not recognize the entry as
being associated with a symbol-level request set from the access
node, the access node can assume that it is another access node's
turn, and does not send a data transmission.
[0140] The assignment message may take on a different format under
an "efficient FIFO schedule mode," in accordance with an
alternative embodiment of the invention. In this mode, each
assignment message may only include one piece of data: an
identifier for the request. The scheduled transmission segment
associated with each request is not directly stated in the
assignment message. Instead, the scheduled transmission segment
associated with each request is inferred.
[0141] Processing of a message under the "efficient FIFO schedule
mode" by an access node such as 104, 106, 108, and 110 is described
below, according to one embodiment. Again, each access node
maintains a local data transmission queue based on assignment
messages received at the first node. The local data transmission
queue may be a first-in-first-out (FIFO) list of all
symbol-level-requests from all access nodes that have been assigned
to scheduled transmission segments, arranged in the order of
transmission. Each entry in the queue may contain just an
identifier for a symbol-level request. The local data transmission
queue is synchronized to the timing of the rest of the system.
Thus, as each transmission occurs or is about to occur, the access
node examines the corresponding entry in the local data
transmission queue. If the access node recognizes the entry as
being associated with a symbol-level request sent from the access
node, the access node knows that its turn has come and, sends its
pending data transmission. If the access node does not recognize
the entry as being associated with a symbol-level request set from
the access node, the access node can assume that it is another
access node's turn, and does not send a data transmission.
[0142] Because a message under the "efficient FIFO schedule mode"
does not contain an identifier for the scheduled transmission
segment associated with the request, it may be more difficult to
keep each access node such as 104, 106, 108, and 110 in
synchronization with the rest of the system. For example, if an
access node misses an assignment message for some reason, the local
data transmission queue maintained at the access node may become
out of sync with the actual timing of data transmissions. To
address this potential problem, scheduler node 102 may periodically
broadcast synchronization messages to the access nodes. This allows
each access node to synchronize its local data transmission queue
with the data transmission queue maintained at scheduler node
102.
Request Detection Error Processing
[0143] Undetected requests are handled by request retransmission,
according to an embodiment of the invention. Here, an access node
such as 104, 106, 108, and 110 may transmit a symbol-level request
that is not detected by the intended recipient, such as scheduler
node 102. A variety of factors may contribute to such a missed
detection, such as noise, interference, etc. When the access node
senses that a symbol-level request it has sent has not been
responded to (e.g., no assignment message is received from the
scheduler node 102 in response to the symbol-level request), the
access node may retransmit the symbol-level request. The
retransmission technique may also take into account considerations
such as retransmission count, latency, and quality of service
(QOS). For example, the access node may keep track of a
retransmission count, such as the number of times a particular
request has been retransmitted. If the retransmission count exceeds
a maximum threshold, the request may be discarded so that no
further retransmission is attempted. Also, the access node may keep
track of a measure of latency, such as the amount of time that has
elapsed since the access node began attempting to request a
scheduled transmission segment for a particular message. If the
latency exceeds a maximum threshold, the request may be discarded
so that no further retransmission is attempted. Furthermore, the
access node may take into account a measure of QOS for a request.
If the request is associated with a higher QOS, the request may be
given a higher priority in retransmission.
[0144] Request collisions detected by scheduler node 102 may be
treated in a similar manner as undetected requests, according to an
embodiment of the invention. Here, two or more symbol-level
requests sent from different access nodes such as 104, 106, 108,
and 110 may collide with one another. Such collisions may occur if
the two or more symbol-level requests are sent in the same request
segment. If scheduler node 102 detects such a collision, it may
simply not send an assignment. Thus, the response from scheduler
node 102 is the same, regardless of whether scheduler node 102
misses the symbol-level request or detects that the symbol-level
request was involved in a collision. That is, scheduler node 102
does not send an assignment.
[0145] Accordingly, the access node does not need to distinguish
between an undetected request versus a detected request collision.
As long as the access node senses that a symbol-level request it
has sent has not been responded to (e.g., no assignment message is
received from scheduler node 102 in response to the symbol-level
request), the access node may retransmit the symbol-level request.
Again, the retransmission technique may take into account various
considerations such as those listed above.
[0146] Request collisions not detected by scheduler node 102 as a
collision may need to be handled differently. Here, two or more
symbol-level requests collide. However, scheduler node 102 fails to
recognize that a collision has occurred. Instead, scheduler node
102 treats the collision as a valid symbol-level request. Thus,
scheduler node 102 sends an assignment message associating the
symbol-level request to a scheduled transmission segment. Each
access node that sent a symbol-level request involved in the
collision accepts this assignment, and in response sends its own
data transmission. As a result, a collision of multiple data
transmissions can occur in the scheduled transmission segment
specified in the assignment.
[0147] The system may handle such a scenario in a number of ways.
In one embodiment of the invention, receipt of the data
transmission is not to be acknowledged. For example, when an access
node such as 104, 106, 108, and 110 sends a data transmission in an
assigned scheduled transmission segment, the access node does not
expect the intended recipient to respond by sending an
acknowledgement (ACK) message to confirm successful receipt of the
data transmission. Here, the access node does not inquire into
whether the data transmission is successfully received. If it is
not successfully received, the access node does not take any action
to re-send the data transmission.
[0148] In an alternative embodiment of the invention, receipt of
the data transmission is to be acknowledged. For example, when an
access node such as 104, 106, 108, and 110 sends a data
transmission in an assigned scheduled transmission segment, the
access node expects the intended recipient to respond by sending an
acknowledgement (ACK) message to confirm successful receipt of the
data transmission. Here, if the data transmission is not
successfully received, as indicated by the lack of an ACK message
from the intended recipient, the access node re-sends the data
transmission. The process of re-sending the data transmission may
be similar to that of the original transmission. That is, the
access node may first send a symbol-level request, then receive an
assignment associating the symbol-level request to a particular
scheduled transmission segment, and finally send the data
transmission in the assigned scheduled transmission segment.
Contention Requests
[0149] According to an embodiment of the present invention, when an
access node such as 104, 106, 108, and 110 sends a symbol-level
request, the access node randomly selects a request segment from
the plurality of available request segments. As mentioned
previously, the request segments may be organized in the request
signal space based on time-division multiplexing,
frequency-division multiplexing, code-division multiplexing, and/or
other multiplexing techniques.
[0150] For example, the request segments can be organized in the
request signal space based on code-division multiple access (CDM).
A code such as a Walsh code may be used. Each request segment
corresponds to a code channel that can be extracted from the
request signal space by applying a particular code word. By
utilizing CDM, the request segment is extended in time and spread
in frequency. That is, without applying code-division multiple
access, the request segment may be confined to a shorter time slot
and narrower frequency bandwidth. Thus, the energy of a request can
be spread out over a longer time slot and over a wider frequency
bandwidth. This allows the transmitter to use less power.
Specifically, the arrangement captures the otherwise unused power
(power in unoccupied time slots and unoccupied frequency
bandwidths) in a low load factor request channel. Use of CDM in the
request signal space also provides a signal-to-noise ratio (SNR)
gain over transmissions in the scheduled transmission signal space.
In certain implementations, this gain can be in the range of 9 to
30 dB, or even more. The use of CDM as applied to request segments
can significantly improve request detection performance.
[0151] FIG. 13 presents a more detailed example of a system with a
contention code request channel, according to an embodiment of the
invention. Here, the system utilizes a scheduled transmission data
length of 800 channel symbols, which is equivalent to 100 bytes in
this case. The average interval of scheduled transmissions from
each node is 8 seconds. The average bandwidth of scheduled
transmissions from each node is 100 symbols/sec. The ratio of
request symbols to scheduled transmission symbols is 8/100. In
other words, every 64 request symbols corresponds to 800 scheduled
transmission symbols. A 64 chip spread code is used is that
provides a 18 dB SNR spread gain. The request channel load factor
is 1/64. The resulting collision probability of requests is
1.55%.
Polled Requests
[0152] According to another embodiment of the present invention,
when an access node such as 104, 106, 108, and 110 sends a
symbol-level request, the access node utilizes a defined schedule
to select a request segment from the plurality of available request
segments. The defined schedule specifies the request segments
available to each access node. In this sense, the defined schedule
"polls" each access node to send its request in the appropriate
request segment. Again, the request segments may be organized in
the request signal space based on time-division multiplexing,
frequency-division multiplexing, code-division multiplexing, and/or
other multiplexing techniques. In particular, the use of
code-division multiplexing in the request signal space is
associated with significant benefits, as discussed previously.
[0153] Generally speaking, a polled request channel has both
advantages and disadvantages when compared to a contention request
channel. One advantage is that there is no possibility of
collision. This is because the defined schedule does not allot the
same request segment to two different access nodes. A disadvantage
of the polled request channel is added latency. When an access node
adopting a polled request channel is ready to send a request, it
might not be able to do so right away. Instead, the access node may
have to wait for the next opportunity when one of its request
segments becomes available according to the defined schedule. Such
delays contribute to the added latency of a polled request
channel.
[0154] According to one embodiment of the invention, the defined
schedule is fixed. For example, the nodes in the system may be
preprogrammed to follow a particular defined schedule of request
segments. According to another embodiment of the invention, the
defined schedule is dynamically altered. For example, the scheduler
node 102 may change the defined schedule of requests segments
during operation, in response to changing conditions. The scheduler
node 102 may then broadcast the newly defined schedule to access
nodes 104, 106, 108, and 110.
[0155] FIG. 14 presents a more detailed example of a system with a
polled code request channel, according to an embodiment of the
invention. Here, the system utilizes a scheduled transmission data
length of 800 channel symbols, which is equivalent to 100 bytes in
this case. The average interval of scheduled transmissions from
each node is 8 seconds. The average bandwidth of scheduled
transmissions from each node is 100 symbols/sec. The request poll
interval for each node is set at 125 msec, which corresponds to 8
request opportunities per second per node. The ratio of request
symbols to scheduled transmission symbols is 8/100. In other words,
every 64 request symbols corresponds to 800 scheduled transmission
symbols. A 64 chip spread code is used is that provides a 18 dB SNR
spread gain. The collision probability of requests is 0% if the
system operates as intended.
Request Signal Designs for Multiple Service Types
[0156] According to an embodiment of the present invention,
multiple services are offered. Accordingly, multiple categories of
scheduled transmission segments are defined in the scheduled
transmission signal space. Each category of scheduled transmission
segments may correspond to a different service type. For example,
as described previously, FIG. 6 presents a two-service TDM request
and scheduled transmission scheme. In this case, there are two
categories of scheduled transmission segments: long request
segments for data transmissions of a first length (long) and short
request segments for data transmissions of a second length (short).
FIG. 6 merely illustrates a simple example. The service types may
be based on factors other than data transmission length. Also,
there may be more than two types of service.
[0157] To request a scheduled transmission of a particular service
type, an access node such as 104, 106, 108, and 110 sends a request
that somehow indicates the service type desired. According to
various embodiments of the invention, such a request may be
implemented as a one-symbol (single symbol) request or an N-symbol
(multiple symbols) request.
[0158] One-symbol requests are described in more detail below.
Here, the access node conveys the service type desired using only a
one-symbol request. According to an embodiment of the invention,
this is accomplished by dividing the request signal space into
multiple categories of request segments. An access node can
indicate the type of service requested by sending the request in a
request segment belonging to the appropriate category. For example,
in the case of two-service system shown in FIG. 6, some request
segments are designated as "long" request segments, while other
request segments are designated as "short" request segments. To
request a "long" scheduled transmission, an access node such as
104, 106, 108, and 110 simply sends a request in one of the "long"
request segments. Such a technique has numerous advantages. One is
that multiple service types (e.g., greater than two) can be
supported. Another is that a flexible allocation of the request
signal space to different service types can be achieved. For
example, in the two-service system shown in FIG. 6, the make-up of
the request signal space, in terms of allocation for "long" request
segments versus allocation for "short" request segments, can be
flexibly changed. Furthermore, this make-up of the request signal
space can be dynamically adjusted while the system is in
operation.
[0159] N-symbol requests are described in more detail below. Two
separate embodiments are presented. According to a first
embodiment, a request comprising N symbols is non-coherently
detected. That is, the recipient of the request (e.g., scheduler
node 102, other access nodes such as 104, 106, 108, and 110)
receives the request as a non-coherent signal. For example, the
recipient may utilize an envelope detector that only detects each
symbol of the request as either an "on" or "off." Generally
speaking, such an N-symbol request can specify one out of 2.sup.N-1
service types. In one implementation where each request comprises
N=2 symbols, the request may support 3 different types of services
as follows. If symbol 1 is "on" and symbol 2 is "off," then service
type 1 is indicated. If symbol 1 is "off" and symbol 2 is "on,"
then service type 2 is indicated. If symbol 1 is "on" and symbol 2
is "on," then service type 3 is indicated. Of course, if symbol 1
is "off" and symbol 2 is "off," then no request is sent.
[0160] FIG. 15 is a plot of expected envelop detector performance
under different noise levels, according to an embodiment of the
invention. According to various embodiments of the invention, an
envelop detector may be implemented to detect requests sent in the
request signal space. This includes detection of one-symbol
requests as well as N-symbol requests, as discussed above. Designs
of such envelop detectors are well-know in the art. FIG. 15 shows a
number of performance curves, known in the art as receiver
operating characteristic (ROC) curves. Each curve represents the
expected performance of the envelop detector for a particular noise
level, which is expressed in terms of a ratio of energy per symbol
over noise (Es/No). Each curve demonstrates the trade off that can
be expected at a particular noise level between (1) probability of
missed detection and (2) probability of false alarm for the
detector. Adjusting the detection threshold of the detector moves
the performance of the detector along the curve. Any point on the
curve represents a particular tradeoff between probability of
missed detection and probability of false alarm. As can be
expected, the overall performance of the envelop detector degrades
as noise level increases.
[0161] According to a second embodiment, a request comprising N
symbols is coherently detected. That is, the recipient of the
request receives the request as a coherent signal. Designs of such
coherent detectors are well-know in the art. The recipient may
demodulate the request according to a particular modulation and/or
coding scheme. One example is a system based on a differentially
coherent detection scheme. Here, M-ary differential phase shift
keying (M-DPSK) modulation is used. An N-symbol request can specify
one of M.sup.N-1 different service types, with one symbol used to
provide a phase reference. Furthermore, the M-DPSK signal may also
be block-coded to provide error correction. Of course, this is just
one example. In other implementations, different modulation and
coding schemes may be used. The particular choice of modulation and
coding scheme may be depend on channel conditions, such as the
signal to noise ratio (SNR) encountered.
Default Assignment of Scheduled Transmission Segments
[0162] According to an embodiment of the invention, the scheduled
transmission signal space comprises a plurality of scheduled
transmission segments that includes at least one default use
segment available for use by a default node or a plurality of
default nodes if not assigned to any symbol-level request. Such a
technique allows for a flexible use of transmission segments that
do not become assigned as a result of a specific symbol-level
request.
[0163] In one implementation, a two-tier assignment technique is
employed. The first tier is a "priority tier" of assignments. This
may comprise an assignment associating a symbol-level request with
a scheduled transmission segment, as discussed in other sections of
the present disclosure. A second tier is a "space available" tier
of assignments. In this tier, one or more scheduled transmission
segments are assigned to a default entity. The default entity is
given conditional use of these scheduled transmission segments. The
condition is that, if the scheduled transmission segments are not
assigned in the priority tier (thus they are available), the
default entity may go ahead and use the scheduled transmission
segments. In other words, the second tier allows for secondary
assignments of scheduled transmission segments.
[0164] Each of these second-tier assignments may be specified for a
particular time duration. Typically, the time duration is
relatively long with respect to the first-tier assignment
activities of the system. Just as one example, in a particular
system, numerous first-tier assignments may take place in a matter
of seconds. By contrast, a second-tier assignment may assign one or
more scheduled transmission segments to a default entity for a time
duration of minutes, hours, or days. Also, the second-tier
assignments may partition the scheduled transmission signal space
in a different way than the first tier of assignments. For example,
a second-tier assignment may assign a large block of scheduled
transmission segments for default use by a default entity.
[0165] In one embodiment, the default entity comprises a default
node. Here, the default node enjoys use of the one or more
scheduled transmission segments if they are not assigned in the
first tier. The default node may use the scheduled transmission
segments to perform file transfers, certain low priority data
flows, etc.
[0166] In another embodiment, the default entity comprises a group
of default nodes. Here, the group of default nodes shares the
default use of the one or more scheduled transmission segments. The
scheduled transmission segments may be allocated as a block of the
signal space. The group of default nodes may share the default use
of the block on a contention basis. For example, each member of the
group of default nodes may compete with the other members for the
right to use the entire block of signal space. Various contention
access protocols may be implemented, such as Slotted Aloha and
others.
[0167] In yet another embodiment, the default entity is selected
from a plurality of default entities including at least one default
node and at least one group of default nodes. Thus, in one system,
the second tier may include a mix of default nodes and groups of
default nodes. A particular second-tier assignment may assign the
default use of a block of signal space to a default node or to a
group of default nodes.
Successive Scheduled Requests
[0168] In response to an initial symbol-level request sent in an
initial request segment from access nodes 105, 106, 108, and 110,
scheduler node 102 may assign one or more additional request
segments as follow-up request segments to the initial symbol-level
request, according to an embodiment of the invention. For example,
scheduler node 102 may broadcast an assignment message to associate
the initial request segment with the one or more follow-up request
segments. This may be performed in an anonymous manner with respect
to the identity of the access node that sent the initial request,
as described previously. The one or more scheduled follow-up
request segments may be used in different ways. Two such uses are
described below.
[0169] In one embodiment, the follow-up request segments are used
to resolve collisions. When scheduler node 102 receives the initial
symbol-level request, it may determine that a collision may have
occurred. Just as an example, assume two access nodes, 104 and 106,
each sends an initial request in the same initial request segment.
This causes a collision. Even if scheduler node 102 can properly
decode the initial request, it remains ambiguous as to which access
node should be the deemed the originator of the request. If
scheduler node 102 does not resolve the collision and instead
simply sends an assignment message associating the initial request
with a scheduled transmission segment, both access nodes 104 and
106 may accept the assignment and attempt to send a data
transmission in the scheduled transmission segment. The likely
result is that the signals will be jumbled, and neither data
transmission can be properly received.
[0170] Thus, instead of just sending an assignment associating the
initial request segment with a scheduled transmission segment,
scheduler node 102 may send an assignment associating the initial
request segment with one or more follow-up request segments. These
follow-up request segments provide an opportunity for access nodes
involved in a collision to send secondary requests that may avoid
collision with one another.
[0171] Continuing with the above example of a collision involving
access nodes 104 and 106, scheduler node 102 may detect the
possible collision and send an assignment that associates the
initial request with 32 follow-up request segments. Access node 104
responds by randomly selecting 1 out of the 32 follow-up request
segments and sending a secondary request in its randomly selected
request segment (e.g., request segment 8). Access node 106 also
responds by randomly selecting 1 out of the 32 follow-up request
segments and sending a secondary request in its randomly selected
request segment (e.g., request segment 13).
[0172] There is a high probability (31/32) that access nodes 104
and 106 would randomly select different ones of the 32 possible
follow-up request segments to send their respective secondary
requests. If this scenario occurs (more likely), the collision has
been resolved. That is, now access nodes 104 and 106 have sent
secondary requests in distinct request segments. Scheduler node 102
can simply process the secondary requests separately--by assigning
a scheduled transmission segment to the secondary request sent from
access node 104, and separately assigning another scheduled
transmission segment to the secondary request sent from access node
106.
[0173] Of course, there is a low probability (1/32) that access
nodes 104 and 106 would happen to select the same one out of the 32
possible follow-up segments to send their respective secondary
requests. If this scenario occurs (less likely), the collision
remains unresolved. Here, scheduler node 102 may repeat the
procedure over again, in order to resolve the collision. That is,
scheduler node 102 may send yet another assignment to assign
another set of 32 follow-up request segments. Access nodes 104 and
106 would again separately make random selections out of the 32
follow-up request channels, and so on. This process may repeat
until the collision is resolved or until a termination condition is
reached.
[0174] While the example discussed above involves two access nodes
104 and 106, the approach is applicable in collision situations
involving more than two nodes. Also, the probability of collision
resolution in each iteration can be modified by changing the number
of follow-up request segments assigned (e.g., to a number greater
than or less than 32).
[0175] In another embodiment, the follow-up request segments are
used to send additional information relating to a request. An
access node such as 104, 106, 108, and 110 may send an initial
symbol-level request. Upon receiving the initial symbol-level
request, scheduler node 102 may send an assignment associating the
initial request segment with one or more follow-up request
segments. Here, the follow-up request segments provide an
opportunity for the access node to provide additional information
that supplements the initial symbol-level request. For example, if
the initial symbol-level request does not indicate a particular
type of service requested, scheduler node 102 may assign one or
more follow-up request segments so that the access node may send
additional signals in the follow-up request segments to specify the
type of service requested.
Piggyback Requests
[0176] According to an embodiment of the invention, a request may
be sent from an access node such as 104, 106, 108, and 110 as a
"piggyback request." Here, a piggyback request refers to a request
that is included in a portion of a data transmission sent from a
node in a scheduled transmission segment. Thus, unlike a request
sent in a request segment in the request signal space, a piggyback
request is sent as part of a data transmission, such as in a header
of the data transmission, sent in a scheduled transmission segment
in the scheduled transmission signal space. This technique may
improve efficiency by allowing a request to "hitch a ride" on a
scheduled transmission, thus obviating the use a separate
transmission in the request signal space.
[0177] Operation of piggyback requests can be viewed in the context
of a sequence of data transmissions sent, in respective scheduled
transmission segments, from an access node. The sequence of data
transmissions may include a current data transmission that the
access node is prepared to send. The sequence of data transmissions
may also include a subsequent data transmission that has yet to be
assigned a scheduled transmission segment. To request a scheduled
transmission segment for the subsequent data transmission, the
access node may send a piggyback request in a portion of the
current data transmission.
[0178] The piggyback request may be included in the current data
transmission in various ways. For example, the piggyback request
may be part of a header in the current data transmission. In one
implementation, the piggyback request comprises a request count in
the header of the current data transmission.
[0179] In one embodiment of the invention, a piggyback request may
serve as an initial request for the subsequent data transmission.
For example, the piggyback request may be the only request sent
from the access node for requesting assignment of a scheduled
transmission segment for the subsequent data transmission. In this
case, there is no separate request signal sent in the request
signal space for the subsequent data transmission. That is, the
subsequent data transmission relies on the piggyback request to
obtain an assignment of an appropriate scheduled transmission
segment.
[0180] The access node may make a burst time decision to select
between using a piggyback request versus using a symbol-level
request in the request signal space, according to an embodiment of
the invention. The burst time decision occurs around the time when
the current data transmission is about to be sent. The selection
may be based on the expected latency of the current data
transmission. The selection may also be based on certain
quality-of-service (QOS) considerations associated with the data
transmission to be scheduled, i.e., the subsequent data
transmission. Just as a simple example, a particular QOS
requirement for the subsequent data transmission may be that
requests are to be sent in less than 100 msec. If the expected
latency of the current data transmission exceeds 100 msec, the
access node may choose to use a symbol-level-request in the request
signal space, instead of the piggyback request.
[0181] In one embodiment of the invention, a piggyback request may
serve as a redundant request corresponding to a previously sent
request. Here, each access node sends not only (1) a request in the
request signal space, but also (2) a redundant piggyback request in
the scheduled transmission signal space. The piggyback request may
include a location identifier for the associated request in the
request signal space. Thus, each request sent by an access node in
the request signal space has a back up in the form of an associated
redundant piggyback request.
[0182] One use of such redundant piggyback requests is detection of
collisions between multiple requests in a request segment.
Different nodes such as access nodes 104, 106, 108, and 110, may
send requests in the same request segment in the request signal
space, causing a collision. If each access node also sends an
associated redundant piggyback request as backup, the redundant
requests may be processed to detect the collision. Another use of
such redundant piggyback request is for detection of missed
requests.
[0183] FIG. 16 depicts use of piggyback requests for request
collision detection and missed request processing, according to an
embodiment of the invention. The figure shows possible events at a
node such as scheduler node 102, and corresponding action
taken.
[0184] First, upon receiving a request in the request signal space,
scheduler 102 assigns a scheduled transmission segment to the
request. Second, upon receiving a single (sole) piggyback request
associated with a particular request detected in the request signal
space, scheduler 102 ignores the piggy back request. Here, receipt
of a single piggyback request indicates that only one access node
sent the request in the request signal space, and thus no collision
occurred. Third, upon receiving multiple piggyback requests
associated with a particular request detected in the request signal
space, scheduler 102 assigns a separate scheduled transmission
segment to each piggyback request. Here, receipt of multiple
piggyback requests that are all associated with the same request
detected in the request signal space indicates that more than one
of the access nodes sent the request, and thus a collision
occurred. Thus, scheduler node 102 resolves the conflict by
assigning each piggyback request a separate scheduled transmission
segment. Fourth, upon receiving a piggyback request for an
undetected request in the request signal space, scheduler 102
assigns a scheduled transmission segment to the piggyback request.
Here, the piggyback request is associated with a request that was
sent in the request signal space, but somehow was not detected.
Here, the piggyback request serves its purpose as the backup
request. Accordingly scheduler 102 assigns a scheduled transmission
segment to the piggyback request.
Dynamic Partitioning of Request Signal Space and Scheduled
Transmission Signal Space
[0185] According to an embodiment of the present invention, the
allocation of the shared communication medium between the request
signal space and the scheduled transmission signal space can be
dynamically altered. Just as a simple example, a system may
dynamically alter the allocation of the shared communication
medium, from an allocation as illustrated in FIG. 5, to an
allocation as illustrated in FIG. 6. As discussed previously, FIG.
5 depicts a particular allocation of the shared communication
medium based on frames of TDM time slots. FIG. 6 depicts a
different allocation of the shared communication medium, also based
on frames of TDM time slots. During operation, a system may
dynamically switch from the allocation shown FIG. 5 to the
allocation shown in FIG. 6. A node such as schedule node 102 may
broadcast one or more control messages to other nodes such as
access nodes 104, 106, 108, and 110 in order to notify them of the
change in the allocation. Once the other nodes have been notified
of the change, the system can switch over to operation according to
the new allocation of the shared communication medium.
Two-Step Request Occupying Request Segment and Scheduled
Transmission Segment
[0186] According to an embodiment of the invention, access nodes
104, 106, 108, and 110 may send a two-step request for
transmission. Such a two-step request is made up of a first step
and a second step. The first step may simply be a symbol-level
request sent from the access node, as described previously. When an
assignment is obtained associating the symbol-level request to a
scheduled transmission segment, the access node can send a data
transmission in the scheduled transmission segment. Here, the
second step may be the data transmission sent in the scheduled
transmission segment. In other words, instead of using the
scheduled transmission segment assigned to a request for sending
regular data, the access node uses the scheduled transmission
segment to send the second step of the request. The second step may
further define the request. The request may be a more elaborate
request for service. For example, the second step may specify a
particular service type or other information that provides
additional details on the request being made.
Multiple Requests for Priority Access
[0187] According to an embodiment of the invention, an access node
such as 104, 106, 108, and 110 may send more than one request when
attempting to obtain an assignment to schedule only one data
transmission. Ordinarily, when the access node has a data
transmission to send, the access node sends one request in order to
obtain an assignment for a scheduled transmission segment. Here,
however, the access node may send out two or more requests. In
other words, the access node may send an extra request(s) as
backup. This may be done for the purpose of increasing the
likelihood of an assignment. For example, if scheduler 102 somehow
fails to detect one of the requests, the other request(s) may still
be detected. For each detected request, scheduler 102 may send an
assignment that assigns the detected request to a scheduled
transmission segment.
[0188] Of course, this can lead to multiple assignments of separate
scheduled transmission segments, reserved for just one data
transmission. According to one embodiment of the invention, the
access node does not waste such extra scheduled transmission
segments. Instead, the access node utilizes such extra scheduled
transmission segment for sending other data transmissions.
No Scheduler Mode and Other Assignment Modes
[0189] Different assignment modes may be adopted according to
various embodiments of the invention. These may include a
"scheduler mode," a "no scheduler mode," and a "hybrid mode." Here,
a "scheduler mode" refers to a mode of operation in which access
nodes rely on a scheduler node to determine the assignment of
scheduled transmission segments. Previous sections have described
examples under the "scheduler mode."
[0190] FIG. 17 presents a simplified network operating under a "no
scheduler mode," according to one embodiment of the invention. A
plurality of access nodes 1404, 1406, 1408, and 1410 are shown that
utilize a shared communication medium. Instead of depending on a
scheduler node to receive requests and determine the proper
assignment of scheduled transmission segments, each access node
1404, 1406, 1408, and 1410 independently determines the proper
assignment of scheduled transmission segments. Here, it is assumed
that all access nodes follow the same rules for determining
assignments, and all access nodes can detect all requests. If this
is the case, then the same assignment of scheduled transmission
segments would be generated at each access node. That is, each
access node would independently generate the same assignment. As
such, there would be no need for a dedicated scheduler node. Also,
there would be no need for assignment messages to be sent. Each
access node would be able to locally determine the proper
assignment on its own. Consequently, a feedback signal space may
not need to be provided for sending any assignment messages.
[0191] A "hybrid mode" is described below in accordance with
another embodiment. Referring back to FIG. 1, under this mode, each
access node such as 104, 106, 108, and 110 receives both (1)
assignment messages from a scheduler node such as 102 and (2)
requests from the other access nodes. Here, each access node does
independently determine the proper assignment of scheduled
transmission segments based on requests received from other nodes.
However, in making the determination, the access node also takes
into account the assignment messages received from the scheduler
node. By utilizing both sources of information, each access node
can make a more robust determination regarding what is the proper
assignment of scheduled transmission segments.
[0192] According to yet another embodiment of the invention, a
system may contain a mixture of access nodes operating under
different assignment modes. Some of the access nodes in the system
may operate under a "scheduler mode." Some of the access nodes in
the system may operate under a "no scheduler mode," as discussed
above. Finally, some of the access nodes in the system may operate
under a "hybrid mode, as discussed above.
[0193] While the present invention has been described in terms of
specific embodiments, it should be apparent to those skilled in the
art that the scope of the present invention is not limited to the
described specific embodiments. The specification and drawings are,
accordingly, to be regarded in an illustrative rather than a
restrictive sense. It will, however, be evident that additions,
subtractions, substitutions, and other modifications may be made
without departing from the broader spirit and scope of the
invention as set forth in the claims.
* * * * *